If your sure your water quality is good(actually checking with a quality liquid test kit) then I'd would assume it's a digestive issue.
The most common signs of Bloat are being reclusive, lethargic, spitting food, stringy white poop, and a sunken belly...rarely do the fish actually bloat up. I would suggest a couple of rounds of Metrozoidale, I use Parasite Clear by Jungle. If the situation progresses then I'll turn to Clout which is a nasty medication combination which is for emergency purposes only IMO. If the fish is spitting food and refusing to eat I'd start treating immediately. It you don't have the meds you can also dose Epsom salts at 1tbs per 5g slowly over a few hours time, until the tank is treated.
During this time of 4-5 days do not feed the fish or turn the lights on as Metrozoidale is effected by light. Treat the tank for 2 days then perform a 30-40% water change and dose again at full strength.
Afterwards the cause must be addressed, the most common reasons are stress, poor food choice as in low quality high protein, and to much salt.
Improper diet is in my opinion one of the largest causes of death in Mbuna.
