I have the same issue with Preston. He is now 15 months and his hair doesn't seem to grow very well either. He has a thick coat, doesn't mat easily (as a matter of fact I hate to say it, but I only comb him on Sat when I do his bath, and then he only gets mats in the hair in his front arm pits) it just seems to grow really slow. I don't know that he will ever have hair that reaches the floor, he is too much of a scrapper for no rough housing
to keep it from breaking. It seems the hair on the top of his head is thinner, not thin, just not as thick as the hair on his body, and really hasn't appeared to grow since we got him. He still has the dark puppy hair on his ends of his body hair since we have never had it trimmed. By the way Preston is a teapot too, weighing in @ 13 lbs the last time he was weighed.