Try taking pictures of them, a lot of times they are growing more than you really think they are but since you see them everyday you don't realize it. Just take a few pics with some reference for size and then in a month pull the pics out and check, you'll see more growth than you think. I've had my JD for a year now and he's 6", got him when he was 2.5, so they do grow slow, but you'll never realize it until you compare how they did look.
JMHO
--Jimmy