Is it only under moonlight or do they not open under regular lighting?
How long have they been in your system ?
I got some z's & p's a wile back most were 2-3 p per frag. It took a few of them more than a month to fully open up and show the whole polyp and skirt , no clue why most softies open up immediately day one but I guess they maybe didn't care for the adjustment.
What test kits have you used to determine your parameters , this factor can make a difference.
If they stay closed up and parameters stay stable I'd consider a dip like the above stated by RacinRevo , the iodine does encourage polyp extension sometimes.
Then again sometimes it just needs time.
Or another theory , perhaps something is aggravating them cuc or pest ?