What possible issues could prevent my dog from gaining weight?

My foster dog is a 3 1/2 year old white shepard mix who is fairly active. When I got her, she was very underweight and I have got her to gain 12 pounds but she was eating 8 cups of hypoallergenic food a day. Now that she is at a better body weight, I tried decreasing her food to 6 cups a day and she lost 4 pounds within a week. I put her up to 7 and she hasn't seemed to gain weight but I have not been able to weigh her. She does eat really fast so I have her eating out of a kong wobbler to help slow her down. Her stool is also quite liquidy. I have been giving her 3 tablespoons of bran which seems to be helping. The vet has tried so many options so I was wondering if anyone else had and overcame a similar issue who could give me any tips or ideas? I am trying to make her more adoptable by decreasing her food intake, so any suggestions would be helpful. I can't, however, change her food as she has some sort of skin condition. Thank you to anyone who tries to help me!