Sensor loses connection after a period of time

Hi everyone, I have two ultrasonic sensors with the ttgo board (v 2.0), and after the initial OTTA, after a period of time they “disappear” from the ttn…everything up to that point looks fine, the battery voltage is perfect, the connections are very stable. A reset will get them back online, but they soon disappear again. I have a number of other devices (including ultrasonic) that don’t have this issue. I thought it may have been a poor snr, and I have tried an abp connection on both, but the same issue occurs. I’m assuming hardware, but is there any known similar issue that I need to consider?
I’m in Australia, on AU915. Using the lmic library. Thoughts?