it could be a simple diagnosis. internal parasitic infections can cause sunken belly. the fish continues to eat, but fails to gain nutritional intake, because of the aggressive competition for it from within the fish. sometimes fecal material will offer clues. passing string type feces, empty stool casings, herniated/inflamed anal tissue, etc. are all potential diagnostic clues. sometimes, there are no visible clues, other than weight loss, and/or the incapacity to gain it. treatment is simple, most shelf drugs are 100% effective, and it's always best to treat the entire tank.
secondary options of diagnosis rapidly become scary. everything from TB, toxic shock, cancer, internal organ failure, etc...obviously, accurate diagnosis can be pretty tricky (and expensive, if not impossible) for wet pets.
i always err on the side of caution by starting with the parasite treatment. an otherwise healthy fish should respond (begin to show gain of weight, feces returns to resemble color of pellet) within weeks of a successful treatment. second round treatments can't hurt, but some antibacterial meds (if added for a 1-2 punch combo) can cause other troubles, if the second treatment is carried out too soon.
in my fish room, if the fish fails to respond, it becomes 'c' graded. aka 'medium risk tank mate, poor visual appeal, inappropriate for breeding' HTH.