If the kitten is the runt or sick the mother may be rejecting it. This is normal. You should buy some formula and hand feed it if she refuses to. Talk to your local vet for more information on how to care for this kitten.
If you think about it this way..all of a sudden all these kitties want to nurse at the same time.id be standoffish too.i have twins and they always had formula not me. Read up or call a vet to ask a few ques.they may have anwsers