My cat keeps puting one kitten out?

There could be something wrong with the kitten. Mothers can sometimes sense the weakest of the litter or something wrong with one in the litter and she will abandon that one. If she continues this and won't nurse I would suggest calling the vet to have the kitten examined. My cat Angel did this with one of her kittens. I kept finding it at the corner of the box without the others and I would put it with them and it seemed it could not nurse. I tried to bottle feed it and didn't seem to have much luck. I was going to take it to the vet but sadly it died that night. Don't delay.