You might try giving the kitten a wee bit of kitten milk from either the vet's office or the petstore. I suggest a supplemental milk called KMR for felines. You might also try mixing a small amount of the KMR milk with a bit of moist IAMS kittenfood...mind you just the smallest amount of the meat/milk mixture , hand fed in tiny amounts to the baby.
I wonder if the baby is anemic? Does she have fleas? You can help the baby to nurse if you still have her with the mother by moving a stronger baby aside and physically helping the baby onto mom's nipple. I've had some succes at this method, though it involves a bit of reassuring/negating momcat's interference and keeping her still long enough for the kitten to eat.
Goodluck! Try the KMR with the dropper method, but be carefull to keep nostrils dry while feeding. If the dropper sems too much, a scrap of cloth dipped into the fluid and pressed to the kitten's mouth can work as well.