calcium may not be your problem, but a range of 400~500 seems to be common among reefers on this forum. I have a fairly new sun coral as well, and it does not extend polyps very much.
I trick it into feeding by blowing a mixture of cyclop-eeze and oyster eggs over the polyps... just use a pipette or baster, and turn your pumps off first.... they probably won't eat the mix... but they'll get a good taste...
8-15 minutes later, the polyps will probably open wide up looking for some more substantial food.... then, go after it with mysis or silversides...
Works for me... on quite a few of my corals, actually... using this technique allows me to feed them when *I* want to feed them... not when they ask for it. Good luck! ...and make sure you have it in the shade... mine seems to retract under bright lights...