The Quick Answer
If your RW402B isn't printing via Bluetooth, check your app pairing. Here are the 7 most common fixes for this model.
The RW402B is one of Munbyn's Bluetooth label printers, and that Bluetooth part is exactly why this article exists. When customers tell me their Munbyn RW402B is not printing, my first question is never about the printer. It's "how did you pair it?" — because the single most common cause of a silent RW402B is a pairing step that looks correct and isn't.
We'll start with that fix, then work through the rest in the order I'd run them on my own bench. Most of these take under five minutes.
Quick-Fix Summary
The rapid-fire version:
- Pair inside the Munbyn Print app, not in your phone's Bluetooth settings.
- Disconnect other devices — the printer only serves one boss at a time.
- Recalibrate with 4+ connected labels if the red light is involved.
- Check the roll — printable side up, guides snug.
- Restart everything — printer, phone, app, in that order.
- Reinstall or update the app if printing died after a phone update.
- Test over USB to split the problem in half.
Fix 1: Pair in the App, Not in Phone Settings (My Uncommon Tip)
I'm putting my uncommon tip first because it solves more RW402B complaints than the other six fixes combined, and almost no guide explains it clearly.
Here's the mistake: you go to your phone's Bluetooth settings, find the printer, tap it, see "Connected," and assume you're done. Then you try to print and nothing happens. The printer sits there, connected and useless.
The right way: open the Munbyn Print app (or whichever label app you use), go to its own device or printer section, and connect to the RW402B from inside the app. That in-app connection is the one that actually carries print jobs.
Why this works: A phone's system-level Bluetooth pairing is just a handshake — it proves the two devices can talk. Label printing runs through the app's own direct connection to the printer. Some phones make it worse: pairing at the system level first can actually block the app from grabbing the printer. If you've already paired in settings, "forget" the printer there, then connect fresh inside the app only.
One more Android quirk while we're here: the app needs location permission to scan for Bluetooth devices. That's an Android requirement, not Munbyn being nosy — Bluetooth scanning can reveal your location, so Android gates it behind that permission. Deny it and the app will search forever and find nothing.
Fix 2: Kick Off the Other Devices
The RW402B holds one Bluetooth connection at a time. If your partner's phone, your tablet, or your old phone paired with it last week, it may still be hogging the line.
Turn off Bluetooth on every other device that has ever met this printer, or power cycle the printer to drop all connections, then connect from the device you actually want to print from.
Why this works: The printer doesn't announce "line busy." It just quietly ignores newcomers while holding its existing connection. In a two-person shipping operation, this is the number one cause of "it works on your phone but not mine."
Fix 3: Solve the Red Light First
If the printer's status light is red — blinking or solid — no connection fix on Earth will make it print. The printer is refusing jobs, not missing them.
Blinking red: recalibrate. Load at least four connected labels, hold the FEED button until one beep, release, and let the printer run paper on its own. Solid red: press both lid corners until you hear two clicks, confirm paper is loaded, and give it fifteen minutes to cool if you just printed a huge batch.
Why this works: The RW402B queues incoming jobs but won't run them while in an error state. Fix the light, and the stuck job often prints immediately — so cancel old jobs first if you don't want a surprise stack of labels.
Fix 4: Check the Labels Themselves
Open the lid. The printable (coated) side must face up as labels exit. Slide the guides so they just touch both edges of the roll. If you're getting label after label with nothing on it, do the scratch test: drag a fingernail hard across a label. Direct thermal paper turns black. No mark means non-thermal paper, and no setting will ever fix that.
Why this works: Thermal printers have no ink; heat darkens a chemical coating. Wrong side up, and the heat hits bare paper. The printer "prints" perfectly — onto a surface that can't respond.
Fix 5: The Three-Step Restart
Restart in this specific order: printer off, phone restarted, printer on, app opened fresh, reconnect inside the app.
Why this works: Bluetooth stacks on phones get gummy — half-dead connections that show as active but pass no data. A phone restart clears the stack completely, which toggling Bluetooth off and on often doesn't. Order matters because you want the printer advertising itself before the freshly-booted app scans.
Fix 6: When a Phone Update Breaks Everything
Printing worked for months, then died the week your phone updated? Classic. Update the Munbyn app from your app store; if there's no update, delete the app entirely and reinstall it, then re-grant every permission it asks for and reconnect.
Why this works: Phone OS updates regularly tighten Bluetooth and permission rules, and older app versions get caught out. A reinstall forces the app to request permissions under the new rules instead of limping along with stale grants. I see a wave of these in my shop after every major iOS and Android release — regular as the seasons.
Fix 7: The USB Split Test
Still stuck? Cut the problem in half. Connect the RW402B to a computer with a USB cable, install the driver, and print a test page.
Prints fine over USB? Your printer hardware is healthy and the problem lives entirely in the Bluetooth/app chain — go back through fixes 1, 2, 5, and 6. Won't print over USB either? Now you're looking at the printer itself: run a self-test page and get ready to talk to support.
Why this works: Troubleshooting is elimination. One test that separates "printer problem" from "connection problem" saves you from blindly redoing every fix. It's the first thing I do on my bench, and it should be the last thing you do at home before emailing support.
When to Call a Professional
Escalate when the USB test fails too, when the self-test page prints with permanent white streaks (physical printhead damage), or when the printer won't power on with a known-good adapter. Contact Munbyn support with your self-test page and a short video of the behavior — warranty replacement on these units is common and their response is quick. Out of warranty, compare any repair quote against a new printer honestly. On a printer in this class, replacement usually wins, and I'd rather tell you that than charge you to learn it.
FAQ
Why does my RW402B show "Connected" but won't print?
Because system-level Bluetooth pairing isn't a printing connection. Forget the printer in your phone's Bluetooth settings and connect from inside the Munbyn Print app instead.
Why can't the Munbyn app find my RW402B on Android?
Usually a denied location permission. Android requires it for Bluetooth scanning. Enable it in your phone's app settings, then scan again with the printer powered on and nothing else connected.
Can two phones use the RW402B?
Yes, but not at once. The printer holds one connection at a time, so the second phone must connect after the first disconnects. Power cycling the printer is the fastest way to free the line.
My RW402B prints from USB but not Bluetooth. Is it broken?
No — that result proves the hardware is fine. The fault is in pairing, permissions, or the app. Work through the app-pairing fix and the three-step restart.
When your Munbyn RW402B is not printing, resist the urge to blame the hardware first. Pair inside the app, clear off rival devices, fix any red light, and use the USB test to corner whatever's left. Follow that order and this printer goes back to being what it should be: boring, reliable, and completely forgettable — the highest praise I can give a machine.
