Don't worry, It sounds like your frog is a little shy. Not all frogs are instantly open to the idea of being tong fed. Crickets can't climb glass, so unless you have a plant leading to the top without a lid, they won't be able to escape. Keep dropping prey in there and I have a feeling you'l see many more dissapear! You can always tong train at a later age, be sure to drop the prey on the floor with the tongs, so tongs will be associated with food.