I'm assuming that the system the polyps are in is well maintained and there is no problems of light, water flow and chemistry. So please make sure all that is checked out.
Keep an eye on the polyps and search more for the nudis at night, or other pests.
Make sure your fishes or other inverts, like hermit crabs and worms, aren't bothering the zoas.
Maybe you don't have anymore nudibranchs, after the first dipping, but continue search for the eggs...
It is best to avoid dipping the polyps too many times "just because".
Perhaps the zoas are trying to recuperate from the dipping (?)...
You need to find out if there is any other predators or irritators.
The point here is that many times people keep dipping the polyps and the problem never gets solved because isn't related to dipping in the first place. To tell the truth, depending on the dipping method it could affect the polyps and lower their immune system!!
Some people dip and don't even know why they are dipping. "Oh, they've said it's good for the zoas". In most cases people don't even know what is that type of dipping for. LOL!!!
Just keep cool and try to solve the problem thinking of what should be applied to the problem.
Some times leaving the polyps alone is the only thing that should be done.
Good luck!
Grandis.