Try goats milk. You can find it in most grocery stores, in cans - condensed

I also make chicken bouillon from cheap chicken backs or thighs and then defone the meat and mix it in - makes a nice nutritious soup kittens like

Try "esibilac". You can find it at pet stores. It is on a similar vein to similac for babies. Warm it in the microwave a bit, and then add it to crushed up kitten chow (crush the kitten chose with a rolling pin in a baggy).

We use to do this at the shelter for litters who's mother would refuse to nurse.

Does the shelter cover any costs? I suggest taking a fecal sample to a vet and having them check for worms. That may be why the kittens are not gaining weight.

To feed them canned food try adding a small amount into their wet/dry food. That way their system can adjust to it slowly. You may also want to try a different brand or type of canned food. Certain brands upset my pet's tummy where others don't.

You might add a little cooked rice with chicken, too.