I've noticed Mr Jingles does the same some days. I put 10 or so crics in his cup and somedays I find them all gone and somedays not. How old is your cham? It could be that he doesn't see the cup. I put the cup on the same spot everyday and I trained him by putting semi squashed bugs on the leafs going down to the cup, so he will go eat those and then see the ones inside and eat them too. Some of the more experience members can perhaps give better advise, but it seemed to work for Mr J. He now knows where the food is
He has eaten out of the cup before during the day. This is the second time he hasn't. I put cup in same place. And then as soon as I put a cricket up to him he ate so fast. I believe his age is around 3-4 months but I'm not really sure.