Door sensor chirps have stopped

All contact sensor chirps suddenly stopped working. They were working just fine since setup over a year ago. No settings have been changed. Firmware on base station is up to date. Contact sensors show open/closed appropriately in the app suggesting the sensors themselves are working. Have done a quick touch of the reset button on the base station but this didn’t fix the problem. Have done a reboot of the base station via the app. Have switched each contact sensor chirp tone to one of the base station only tones. Have tested the siren on the base station (and it works).