Have you talked to the breeder about your problems? Usually it is best to turn to the Breeder for behavior problems and especially to see if this is a common problem with the litter. How old was she when you brought her home? My puppies do not leave their littermates until at least 10wks. It is during the 6 to 10 wk period that play with their littermates teaches puppies to have a soft mouth, usually.. or playing with adults, as they are quick with the corrections. There are lots of posts on the Forum about mouthy puppies and ways to curb…
What are the situations when the puppy becomes mouthy? What are you/she doing? During play? Getting too excited... getting over tired..?
And as another note, where the parents of that pup DNA tested for Fanconi?