I own a bearded dragon, mine did not get any longer by the time he was about 13 months. He is on the smaller side only about 20 inches and weighs about 550 grams. The lesser weight could be due to the dragon being mistreated but it might have residual problems like a parasite and that could be why it is not gaining weight like the other. I would say to take the dragon to a vet or at least a poo sample (most vets can do a poo sample, but to take it in I would take time to find a good exotic pet vet). a good website to check out is beardeddragon.org and you should review your husbandry as well. Hope this helps!