Yes that's been me with all three, dd1 was on solids at 4 months we worked out at 8 mths she was lactose intolerant when she self weaned onto lf formula. I know it's rare but ds is the same we have worked it out with a process of elimination with him, even a2 milk upsets his tummy.
Dd2 was a bit different she was just tiny and took a while to learn to attach and was fine from a fw months old.

Dd1 is very active which doesn't help with her weight gain at three she is still very slight. We encourage three meals and two snacks a day if we can get her to eat it. She was a premmie and was monitored by a pead.

Ds has reflux which isn't helping his weight gain, he is on Zantac now and it's improving but his pead wants him to average 200g a week he is only at 60 ATM, so we have a way to go. Unlike dd, ds doesn't appear to throw up much so it's confusing when he feeds a lot but won't put on weight.

Don't know if I have helped much but I have an currently going through the same thing.