DIAGNOSIS The printer's Bluetooth module is not in discoverable mode. The most common cause is a corrupted pairing list in the printer's memory.
FIX Clear the printer's Bluetooth cache. Turn the P21 on. Press and hold the Feed button and the Power button simultaneously for 5 seconds. All LEDs will flash. Release the buttons. The printer will reboot. This clears all paired devices. Open your phone's Bluetooth settings and delete any existing "Nelko P21" entry. Search for devices again. The printer should appear as "P21-XXXX".
IF THAT DIDN'T WORK The issue may be with the phone's Bluetooth stack. Power cycle your phone. Then, on the P21, perform a factory reset. Turn it off. Hold the Power and Feed buttons for 10 seconds until the screen shows "RESET". This resets all settings, not just Bluetooth.
HEADS UP The P21 only supports Bluetooth 4.0. It will not connect to devices using Bluetooth 2.0 or 5.0-only modes (like Android's "Nearby Share"). Ensure your phone's general Bluetooth is enabled, not just a quick toggle.