Zero byte files

I have two new audiomoths (the most recent version). I've been testing both of them out for the past month. Both were working fine, but now both have started recording zero byte files.

Following advice from another post, I reformatted the cards (in two different windows computers, tried exFat and FAT) but am still getting the same problem. When I set up a short recording and watch, the unit blinks a strong green when recording starts, a couple of reds, then goes back to blinking green. The file created has the right time stamp, but zero bytes. I'm not removing the card prior to the recording finishing.

The cards are 32GB Kingston. I can think of a long list of other things to try (new cards, new audiomoths, etc.) but before I go down that route, I wonder if anyone else has encountered this issue (because it is quite possible I've just started doing something silly, and don't realize it).

Many thanks, Phil Taylor.