If your kittens are not nursing you may have to step in and feed them, Get a kitten formula for this, do not use cows milk as it can cause diarrhea and make matters worse. You will also need to stimulate them to pee and poop with a warm cloth by gently rubbing the genitals and anus, this mimics the mother cleaning them.
If mom is cleaning them then let her.
They need to be in a warm, clean place where the mother is comforatable
The best way to know for certain if there are more kittens in her is to have her checked by your vet, they may want to do x-rays to be sure.
Take kittens also to have them looked at.
Unfortunately some puppies and kittens just don't do well, they fail to thrive for whatever reason whether it be genetic or underdeveloped organ systems and no amount of care can change that.
I hope that is not the case, and I certainly don't mean to sound discouraging.
Best to get them in for a good check-over.
While you are at the vet you may consider asking him/her about the benefits of spaying her. The pros far outweigh the cons.