Kitten food can often be too rich for young cats. The vet said that my young nursing mother cat should be fed kitten food (as well as her kittens once they were weaned) in order to provide her all the nutrients that she (and her kittens) needed. It was an absolute disaster, with Mum having a permanently upset stomach.
Then the lady who runs a local burger van told me that, over the years, she'd had loads of cats and loads of kittens. She'd only ever fed them normal (canned) adult cat food, simply mashing it up into small pieces for the youngsters, with every one of them thriving.
So, since the vet's advice simply didn't seem to be working, I followed the totally unqualified (but very experienced) advice of the burger van lady instead and fed both he adult cat and her kittens on adult cat food. The problem was solved straight away, with Mum being a much happier cat and her kittens thriving too.
So my suggestion is to stop using the kitten food altogether and see if it helps.