My nephew has been very slow to gain weight, in fact has gained not much more than a pound in the last 6 months. He is not the best eater in the world but is still being breastfed, so I'm sure that he is getting enough to eat.
He is meeting milestones fine, and crawled before he was 6 months old.
He has been reffered to a paediatric doc to get to the bottom of why he is so tiny. They have checked for a murmur but I think they want to check further in case there is something wrong with his heart