We had problems with our little guy gaining weight in the first five weeks. He lost an oz here and there but never any large losses, but he also never really gained. After the third week of no gain I started pumping after feeding him and my mother would top him off with the pumped milk (usually around an oz). After doing that for a week he gained about 9oz is 6 days, and my supply was also increased. We then laid off doing that and he went back to no weight gain. Around that time I also had been seeing an IBCLC for latch issues and she actually diagnosed him with posterior tongue tie based on signs while he was feeding (gulping, clicking, nipples that would not heal despite seemingly good latch, not opening mouth wide, etc).
My ped had declared that he was not tongue tied, but I decided, based on the LC's advise, to go directly to an ENT specialist. He said within seconds of opening my son's mouth "Oh yeah, that's a tie." He told me that my son would probably be ok without clipping it, but down the line he could have speech problems. So I decided to have it clipped and seriously, after that we have never had any problems since. He started gaining, and has stayed on the 15% curve ever since. Do you have the ability with your insurance to go to an ENT directly, without referral? It might be worth the co-pay to just get that checked out. And, as an aside, my son didn't even cry when the tie was clipped, which was great because I was scared it would hurt him.