I was also reading about possible parasites? I think if you have the money than you should definitely have a vet examine the feces before you buy another frog.
Here is a website that suggests using flukers repta-aid and it says it stimulates hunger FROG FEEDING FEEDING FROGS AND TOADS OFFERED BY Pollywogs World of Frogs
Some people have personal negative opinions about flukers, but I think looking around at these types of products may help with appetite on top of what you are doing.
When my chameleon was sick I gave him rescue cal which is packed with vitamins, but it is a stressful process for the animal because it is a liquid that you have to get them to ingest by a syringe so I woud not suggest that for a frog