These guys are susceptible to changes in water quality which they make the mess themselves and one should be prepared to change water frequently testing often as they produce enough waste to pollute water and raise nitrate quickly. Weekly changes of 20% to keep up with trace elements and water quality is noy unusual.
While some looked filled in body contour, pic below is typical. Again, NEVER buy a fish in a shipping bag as poisoning and gas exchanges are the biggest risk. Most stores will not offer a warranty in shipping bag for this reason.
Some foods to entice it to eat are live ghost shrimp or a couple of guppies. Signs of internal issues are hard to distinguish in many cases.
Assure both Salinity and nitrate are not elevated. If this is a brand new tank, may be a contributor.