Are there any signs of tissue recession?
You can try coaxing the feeder tentacles out by temporarily relocating the sun coral into a small container with tank water and adding mysis shrimp or cyclopeeze and keep it in there for 15-30min. If the feeders come out then feed the coral, if they remain retracted, return the sun coral to the tank and repeat daily.
With healthy tubastraea feeder tentacles extend daily, usually at night but also when it senses food in the water (polyp extension occurring within minutes of food introduction or even prior to the food being introduced if the tank is being at the same time each day).
When I first got my sun coral it didn't extend feeders for maybe 3 or 4 days but did so immediately when I tried the above technique.
Rift Lake and Reef tanks.