they do it for many reasons, mostly i find they do it when its time for a water change....but they do it as defence,if theres too much light ,if algae is growing on the polyps,if they are being picked at or handled,sometimes they dont acclimate well zoas are picky like that
are these the ones i gave you??
its perfectly fine for zoas to stay closed for a few days ive had frags i got off people that didnt open for weeks, i got a little frag of pat last year that i got with just a few heads of yellow and orange and they sat in my tank for months before any sign of life.alot of the smaller frags i get from red coral usually take a few days to open and become comfortable.and i buy 5$ frags from ken and i buy the ones that arent open and see if they open up in my system
water changes are a big thing with zoas even if your water tests fine they need regular water changes plus you need to feed them i know alot of people dont but they do a million times better when fed, and mine stay open all night the only reason mine close now is if i need a water change or aiptasia is growing out of them which seems to be my big big big problem lol
try to stay clear of deepwater zoos untill you get the hang of taking care of them , they are hit and miss for everyone