You're not being stupid!
I don't have a solution for you, but it's likely to be the same problem as on my Vivitar 5385. There's an internal low battery sensor in the camera. If this is set too high, then you'll only get the peak initial charge of the battery before it tells you the batteries are flat. They're not. They've probably lost no more than 5% of their charge and will work fine in anything else.
Unfortunately, this will definitely be a factory setting. I'm not even sure if it CAN be changed, but I'm trying to find out.
Peter Johnson, June 2011
