Based on what's been reported here, September is statistically the worst month for missing
Castaway Cay. BUT...that is heavily affected by last fall's hurricane season, during which three different storms hit the Atlantic coast of Florida (and the Bahamas), and due to island damage, there were several cruises which did not even attempt to dock. If you take away last year, the ships are 30-31 in successfully docking in September. I would not hesitate to cruise in September, even considering these numbers. In fact, we are sailing on the 10th this year.
Besides September, the worst month is February, with an 88% success rate, followed by December (92%), January (94%), and October (95%). Overall, the ships are able to dock around 95% of the time. I guess if this is a big concern to you, then avoid February. I personally wouldn't let these numbers decide my vacation date, if February fit my schedule and price range. 88% is still very good odds, I think. Both ships did miss once this year in February, but the Magic also docked 3 times, the Wonder 5.
Hope this helps.