I have found Clethra to be slow growers in my yard, and none of them have taken off quickly. Also, in general I find that shrubs spend their first few years in the ground growing roots and settling in, and only in the fourth or fifth year do I see them start to put on a lot of growth.

Its another acid lover that does quite poorly in my yard. I've lost several of them over the years. It doesn't matter how much sun or water they get, the just don't like it here.