I have/had the same problem with Nikka, she's a pointer/chow but has a pointer body so she's extemely lean as it is. When I first got her she was severely underweight. We went through a few different foods (she's really finicky, she'd eat a little but if she didn't like it that's it). I finally switched her to Canidae (not the highest calorie food on the market) but she loves it so tends to actually eat more. I fed her at will , she could just graze periodically all day and supplimented some canned food in the evenings for dinner, which helped her get to what would be considered normal. She will always probably run a little underweight because of her energy level and metabolism, but she's only 1 or 2 lbs at most. We still have the problem sometimes when it's hot she doesn't eat much, so I always try to feed as late as possible during those times and coat the dry food with wet food or gravy.

As for the breeder, what was her life like before you got her. Did she get as much excersize? If you add additional excersize to a high metabolism then your probably going to get a weight drop initially while she adjusts. Explain the problem to the breeder and see what she suggests. I'm not familiar with this breed but, this can't be her first dog she's had this problem with if the breed is prone to high energy/metabolism.