Some frogs dont want to waste their energy for small prey and prefer bigger food, others are just plain lazy. That is provided they are healthy. You mentioned a frog was fed on large prey only? Is it right? He may have developed certain conditions if a prey was too big.

How about we'll start from beginning. Would you please answer to trouble in the enclosure questions here to start with pictures of the frog. Since you have another one i strongly suggest you keep your new one under quarantine conditions.