Well, assuming all your parameters are good- they will occasionally close for a day or so to shed off a layer of crud. If it does it too much I would suspect that it could use a bit more flow directed towards it.
Enzo, if you haven't already, take a look at our library (IIRC, Advanced Aquarist is archived there as well) here http://www.reefs.org/library
Also, go to http://www.wetwebmedia.com and look up the marine aquarium articles, under the section "Non-vertebrate life". Scroll down.