10 month old pit mix can't gain weight

We adopted our pup in June when she was 4.5 months old- generally a good acclimation with no early health issues, just underweight. Since then, she's grown a bit but not progressed on the body scale at all. She is a great eater and we've gone to a high fiber/high absorbtion food. After a typical bout or two with giardea, she's had plenty of fecal tests showing she's now all clear, is a bit vulnerable to diarrhea without proviable paste, and had completely clear GI and blood screens. Now the vet is recommending going surgical which just feels wrong. We're looking around for a second opinon and I'd really love any ideas on questions or trials we might not be thinking of.