diferent fish have diferent growth rates. As far as clowns go I am not familiar with theirs, but if you are feeding them properly they will grow.
Keep in mind you see them everyday so you won't always notice the growth. In my tank I don't see their growth but when people come over they notice the diference. This is because they don't see the tank everyday.
I have had my clowns for about 3 1/2 months, they were really little, they have almost doubled in size. They are in a 55g, 3 of them share the tank with a algae blenny, coral banded shrimp, a couple of hermit crabs and a new sebae anemone.
I had a pair in a 55 gallon and in 1 year they doubled. Now in my 125 they have grown to about 2 1/2". I bought them together and they were about 3/4" long.