Spotlight Pro not charging with solar

I have a spotlight pro connected to a 4 watt solar panel, but I can't get it to reliably charge the battery. From the Ring app, under device health, it shows that solar is detected. Because it's USB-C I've tried connecting a USB power meter, and it will show it's charging at around 3W or so, but in less than 30 seconds it drops down to 0W. In order to get it to charge, I have to take the battery out, and put it back in and then it will charge for the rest of the day. The next day though, it will go back to the same behavior of not charging until I pull the battery out and re-seat it. I've tried different batteries, and switching from position 1 or position 2, but it doesn't make a difference.