What they said
How long had he gone without eating before you tried force-feeding? That should only be as a last resort and, if stress is the problem, it only makes it worse. I personally wouldn't try to force-feed until the frog started to look thin (after peeing, since they can look obese while starving due to holding water).

The waiting before eating can also be a sign of stress. If mine gets spooked by something (you know, something really dangerous, like a door closing down the hall :P) at feeding time, he takes about twice as long to calm down and actually eat. How do you feed? If you can just leave the food in there awhile and not be present, sometimes that helps.