I believe I would have a fecal done, JUST to be on the safe side.
It could be something as simple as not large enough prey, but it could also be very serious.
I will say that I've seen personally a ball that was fed what seemed an appropriate size prey but was only maintaining weight and barely gaining.
When I got over my unease and offered larger prey, he started gaining steadily, and proved that ball pythons really are made of rubber.
Gale