RealChange,
unfortunatly you hit apon a topic that really doenst not have an answer.
you can ask it of 20 peopel and i assure you that you will find at lesat that many answers.
there are so many things that "might" contribute, but the fact is to my knowledge no one is really "sure"
i have heard all of the items listen above and more
many are contradictory,
like low nutrients or high nutrients
too much light, or not enough light.
too much or not enough calcium
even time of day/night could affect this
this is why you see such a variation in coral.
you can take one beautiful
Acropora,
frag it, put two
frags in the same system
now you have the mother colony and 2
frags.. you could end up with all three looking the same, or three different looking corals.
i have moved corals from one place to another and seen they change
i guess what im really getting at is dont worry about polyps extension, as long as the coral is healthy and growing (degree of extenion is not always an accurate indicator of health)
once you are sure you have that, just make small tweaks to the system or positioning and see how the specimen reacts, each coral is different. what works for one, wont work for another..
i hope this doesn't discourage you, i don't mean to say the answer is not out there, im just saying in my years i have never seen anyone give it from a facts (with scientific testing vs anecdotal standpoint