Sometimes a goat will disown a kid for reasons only she knows. I had one doe that had two kids, a buck and a doe. She thought the buck was wonderful but she picked up the doe kid by the ear and threw it as far as she could.
You have several alternatives. You can milk the doe and feed the kid its mother's milk from a bottle. You can tie her up very short, hold her, and put the kid by her teat so he can nurse.You can feed the kid milk or replacer from a bottle. Plain old cow's milk from the store works better than replacer. You can remove both kids and put them with her only at feeding time. Tie her up very short and put both kids on her at the same time so they can nurse. I would say do this about 4 times a day. After a few days of this she may accept the reject.