Flow and light intensity play a role as well.
Not having the polyps falling out of every piece in not necessariliy a "bad" thing. Often pieces that have transparent or white polyps only come out at night, my 30K and Eddies Purple monster are like that.
Flow plays a big deal with milli and prostrata, too little and they don't come out to play.....I moved my Rasberry Blueberry whatever its called the other day 2" up and two inches over, just by that little move I notice it has less PE, positioning is part of PE extentsion just as it is for coloration.
Light intesity, too much I notice they will retract during daylight hours, this is what I think is my problem. My oregon tort does not have any PE on the lit sides, but blooming in the shade. When my MHs go off and my Actinics are still on, I see increased PE just from that.
The biggest thing is the water quality as mentioned, some SPS are just more sensitive than others and if your params are shifting or off then that effects it greatly.
If your coloration is fine, and your pieces are growing, and they are extended at night, I wouldn't worry too much and overtime as the acclimate to the conditions of your tank, they may change thier mind.
Feeding plays a role as well, in nature most SPS are nightime feeders and PE extenstion during the day for some specie takes a bit of "training" by feeding regularly at the same time during the day. If there is a reason for them to be out, they are more likely to to do. On Tyrees site he mentiones the polyps on the Becker Tort were first white in captivity, but changed to blue over time......this could be an example of that happening.
Some fish nip and pester the SPS......Angles and crabs can make the retract during the day.
You not what SPS stand for right? (S)tability (P)romotes (S)uccess
I think you are starting to see why the call SPS an obsession, you constantly obsess on how to get to that next level or fix whats wrong.........in the end, driving yourself crazy. Keep that in mind, as for me.....well.....:wildone::wildone::wildone:
Like I'm one to talk 
I'd get your system dialed in before you keep adding pieces. my $.02
Id adjust your water chemestry first and foremost, and give things a week to adjust to proper water conditions. 
