Normal.
They kinda take a while to spread in the beginning.
I think most of the blue zoanthus are like that. Many people say they are "hard to keep".
Sometimes other colors of Zoanthus are like that too.
And sometimes if you put many frags from the same colony, regardless the color/species, in different places, in the same tank they will grow in a different rate too (water flow and light).
I would just live it alone and wait. As long as it doesn't have anything eating it and no bacteria infection it should be fine.
One day they begin to go crazy and reproduce fast.
Keep the parameters constant and try to feed a good quality coral food once a week, very small amounts to see what happens - target feeding. Every time I've tried that worked somehow. I believe there is a stimulation for the zoanthids' internal muscles and natural wild behavior.
Most of the time we think they're not growing, but they are.
Normally the less polyps per frag, the slower they'll grow too (not a rule!).
I like the green zoas!
Good luck.
Grandis.
