Ok your husbandry seems to be spot on. I might consider switching the substrate to plain moist paper towels as it may be easier for them to spot the prey items and it can give you an idea as to whether or not they are defecating. Other than that there isn’t much else to say. If you can get a fecal sample to a vet that may give you an idea if parasites are a factor or not but I’m thinking stress is the issue. Stressed animals can be very hard to bring around. As far as force feeding goes it can be done but it must be done very delicately. You can restrain the frog on its back while wearing moistened vinyl gloves. Use a guitar pick or something similar, begin at the side of the mouth and gently pry it open. Introduce a small prey item and don’t force it. The caveat though is that force feeding may in itself stress the frog further so I’d use caution. I do have to ask though how you managed to get this species b accident? Who was the vendor?