I have a 2-week old pup that isn’t gaining weight. I let her nurse by herself 3 times a day. What should I do?

I have a litter of ten puppies that was born two weeks ago today. Nine of them are doing great. The tenth one is confusing me. She appears healthy. Her coat looks good, she appears to be nursing fine (without the other puppies). The other pups push her away when they want to eat. I have not seen any sign of the milk coming back up after she has nursed. She does not have cleft palate. The problem she is not gaining weight as quickly as the others. They are all gaining 1 1/2 - 3 oz per day-she is gaining every day but it is an average of .4-.6 oz per day. I have just started weighing her two times per day and I am letting her nurse by herself three times a day and I am sitting with them to make sure she is feeding-hopefully this helps. Do you have any ideas or suggestions?

Thank you,
Karen