Welcome to reefcentral!
What are you trying to feed it and what time during the day?
Suncorals are non-photosynthetic. Therefore, they don't naturally open up during the day, they open up really nicely at night (unless they were in poor conditions in the previous owners care=if they aren't healthy, they usually aren't able to open up again and require excellent care to bring them back)
But they should open up if you add something like cyclopeeze or phytoplankton or stuff like that. Only try to attempt to feed them when their tentacles are extended, feeding them when they're closed isn't going to do much.
Feed them mysid shrimp (frozen usually) or cyclopeeze, or live brine shrimp if you have the time.
If you have anymore questions just ask, and good luck!
and it might not be dying, a picture might help