That seems a little odd. Considering the breed & the circumstances I'd say the breeder should allow you a little more time, especially seeing as you are certainly trying.
If you can't get her to gain the weight by then try talking calmly to the breeder, explain that you are trying different diets, trips to the vets (offer proof from the vet), exercise regimans, etc...
If I remember right you haven't had her that long either, it takes a little time for the dog to get adjusted to everything & what not. So I'd bring that up as well.

How old is she? If she is under 2 explain that she is still a puppy therefore needs more time even yet.

Offer a compromise or maybe a contract with the breeder. Like ask if she will allow you one additional month, or if your having a real tough time getting her to gain wait (and especially if she is young) ask for 6 months AND in that time you will provide the breeder with info/updates from the vet, receipts from diets, weekly updates on progress etc... Tell the breed that if you fail to provide updates that she can then take her back when ever she feels like it.

Good luck!