I have now dealt with failure to thrive issues with two children and the solutions to each of them have been completely different.
Sparky, now 11, was diagnosed with celiac disease at 19 months. Once we put him on a gluten-free diet he put on weight. However, he did not get any taller. Some months later we learned that he had pituitarian dwarfism. I know it sounds awful, but it's easily treatable -- he gets a growth hormone shot each night.
Fast forward a good number of years and I now have a 7-year old who has dropped from the 50% for height to the 5% over the past four years. He doesn't have celiac and he doesn't have hormone issues. What he has is great big, gigantic tonsils. Yep, humongus tonsils can keep your kid from growing.
His tonsils are so big they cause him to snore like a big grizzly bear. Ever since he was a little baby. I'd mention it to the doctor but he never thought much about it. At least until the kid started dropping off the growth chart.
Anyway, the snoring keeps him from sleeping and if you don't get enough REM sleep then you don't grow. It can also cause learning delays which may explain why the child struggles with his phonics lessons. Also, I'm told it may solve his picky eating problem. Whoda ever thunk? Apparently, it's uncomfortable to eat with tonsils that almost touch each other. We're even told that his voice will change and he'll probably lose his baby voice.
Gosh, it almost seems that removing tonsils can solve anything and everything. I wonder if it'll solve the bickering-with-his-brothers problem. Hey, I can hope!