I have a Dumpy White which hasn't eaten in several days. I managed to force-feed a cricket around four days ago but he still refuses to eat (using a credit card has proven difficult.) The strange thing is he appears very active and looks healthy. He doesn't appear to have any sort of discoloration as far as I can tell. But during feeding time, he shows no stimulation by live food at all (not even looking at the movement of the crickets, and attempts to climb out.) It's winter where I live, but he has never shown signs of hibernation or loss of appetite around winter over the 9 or so years I've had him. Some years back, I had this same problem but after force-feeding him a few times, he got his appetite back.

My main questions are:
Any ideas on what the cause could be? He appears healthy in every way besides completely losing his appetite. I'm at a loss but would like to at least know the cause if the worst happens.
Otherwise, are there any ideas to stimulate his appetite from home? Or something that can be purchased at a local pet store that might increase his appetite?