Stops recording at 4.0V. Doesn't use charge from external battery.

My audiomoth stops recording when the battery goes below 4.0V.

I configured my audiomoth to record for 30 minutes at a stretch with 0 seconds rest between. I am using alkaline batteries and an external battery pack connected via USB. The setup recorded perfectly for 2 days, but after that it started making empty sound files.

I thought maybe the issue was that I hadn't left enough time between recordings. I read that sometimes when the battery is low it takes longer for the audiomoth to write a file and will skip scheduled recordings if there isn't enough of a gap between them. I reprogrammed the audiomoth to leave 30 seconds between recordings but it didn't fix the issue.

I also noticed that the audiomoth is only using charge from the 3 AA batteries and not from the external battery. The external battery is still fully charged, but the audiomoth wont record. When I plug in the battery the green light comes on, the way it does when I plug it into the computer. Could it be that it can't recognize a battery connected through USB?