I've had a similar problem with one of my frogs. When I got him he ate 3 crickets in about 3 weeks, the next 3 weeks he didn't eat at all and I was more than worried. The guy that owns the shop I've got them from (he knows a lot about them) told me that as long as she seems healthy and is not losing weight, it's okay I should not worry too much. Amphibians seem to be able to live without food for months. Mine is eating now as much as the others do.

As the others said I would not force him, it seems that he is able to eat and he just doesn't want to... Maybe he feels a little bit stressed, so I wouldn't take him out for a while (I did it like that with mine).

Another question: You wrote you kept them in a 20 liter tank, but I really really hope you meant 20 gallons..? (If it really is 20 liters that would be the same as keeping a child in a closet and I could imagine why he's not eating.)