Sorry to hear of your frog's condition. According to your post, you already taken your frogs to the vet and they had a clean check-up. I have seen a condition called the "short tongue syndrome" in which frogs are unable to grab prey. When I was working at the zoo as an amphibian keeper, I was in charge of maintaining a critically endangered toad, Bufo baxteri. This species is likely to have "short tongue syndrome". Each week, I administered Vitamin A drops to the backs of each toad. The reason was to keep their immune systems from deteriorating by bacterial and fungal infections. It's a long shot, but you may want to talk to your vet about the effects of hypovitaminosis A on frogs. Good luck and hope all goes well.

Here is an article that you may share with your vet:
http://www.ivis.org/proceedings/navc...E/646.pdf?LA=1