Ok here is something to think about ..
Polyps can close for a period for no reason at all ..then one day just open back up ..
Closely inspect the polyps for anything such as sundial snails or nudis that could be eating them or irritating them also look for any small crabs (not hermits ).. Some folks have had issues with pods
irritating them ...Volt meter if you do not have one get one , you may have stray currents irritating them .
Couple questions for you
Where in the tank are they located ?
What is the tank temp , what is the swing like ?
Tank mates include every thing from corals to fish to inverts?
What is your lighting and cycle of lights ?
How old are your light bulbs ?
What kind of salt mix are you using ?
How old is the tank?
What is your water change schedule ?
What is your Iodine/IodIde levels ?
What about your clean up crew ?
In the mean time you can move them up in the tank if that is possible , you can also take a turkey baster and blow them off you can take them out place them in a bucket of old tank water and just hit them hard with the current to remove anything that could have built up .. then replace them back into the tank ..it is not likely the NitrAte levels as that is not that bad but as you do your water changes these should start to drop off even more ..