You didn't mention two of the most important #s in my book... temp and alkalinity.
Also, you say your numbers are fine, but what are they exactly? What were they last week? Some people will provide alk/calc measurements AFTER they have adjusted them... we need your average daily numbers.
If my alk drops below 8, my zoa growth pretty much stops.
Do you have any algae growth? Cyano, etc? If there's algae growth on the colony or frag rock, that will also stop growth.
Are your polyps open or closed? Are they stretched out tall or short and stubby?
What type are they and is slow growth 0 new polyps per month, or 10 polyps per month?
Pictures are worth a thousand words also....