Kitten not gaining weight

I recently adopted 2 kittens from the Humane Society. One is a tabby and is 4 months old, the other is an Oriental mix and is 5 months old. The Oriental mix seems very frail and doesn't rough-house like the tabby. She is very dainty and slow moving. I am feeding them Taste of the Wild canned food 3x/day (3 cans total between the two of them) and the Oriental mix eats alot, including the Taste of the Wild dry food, which I leave out at all times. My question is this: she doesn't seem to be gaining weight since I got her, she has a big belly while the rest of her is very thin (I can feel her ribs, spine, etc), and her belly makes loud noises right after she eats. Do you think she could have tapeworms? If not, what do you think could be wrong? She is just finishing up medicine for a severe bladder infection and I want to get her completely healthy. Thank you for your help!  
Denise

[Aleda M Cheng, D.V.M., C.V.A]

It does sound like worms.  Gastrointestinal parasites are intermittently shed and don't always show up on fecal tests.  Tape worms are particularly difficult to find.  Drontal Plus or other anti-roundworm and anti-tapeworm medication could be given and if she improves and her belly becomes normal you will have your answer.  If not, further diagnostics are required.