DIAGNOSIS The printer's Bluetooth pairing cache is full or corrupted. Most devices store only a few paired device profiles.
FIX First, clear the printer's Bluetooth memory. Turn the printer off. Hold the FEED button and power button simultaneously for 5 seconds until all LEDs flash once. Release. This performs a factory reset on network settings only. On your phone, go to Bluetooth settings and 'Forget' the printer. Power the printer on. Put it in pairing mode by holding the PAIR button for 3 seconds until the Bluetooth LED blinks rapidly. Pair from your phone again.
IF THAT DIDN'T WORK The issue is likely your phone's Bluetooth stack. Force stop your phone's Bluetooth app (often called 'Bluetooth Share' in Application Manager). Clear its cache. Restart your phone. Attempt pairing again before opening any other apps.
HEADS UP After a successful connection, always send a test print from your app's settings menu before printing a real label. This verifies the data channel is open.