if the apexes were newly installed, give the acans a day or two to revert back to normal. if they've been in a few weeks and now just showing signs of closing, i'd start diagnosing another problem, perhaps a disease or lack of feeding. I feed mine 1x-2x a week mysis per head and they flourish. they also do like a medium flow area on the substrate so food can fall into their mouths...perhaps a different location?
I have a few and they've all closed up... I installed them a couple of weeks ago... I think I'm going to dip them all and see if they react better after that