Have you observed your sun coral after the tank lights have been off for 2 or 3 hours? Tubastrea is typically a nocturnal coral.
If your sun coral is not permanently affixed in your tank, get a small container and add enough tank water in it so that when the sun coral is temporarily placed in it you've got about 2" of water above it.
Add some mysis shrimp and/or cyclopeeze to intice the polyps to extend. It may take 5-20min for it to do so. If the polyps have extended feed the coral the mysis shrimp and cyclopeeze. If there is no reaction, return the sun coral to the tank and try again the next day or so.
If the sun coral is permanently affixed to the live rock in your tank and cannot be removed use the top of a 2-liter soda container and place that in the tank over the coral so that it forms a dome over it; then and introduce the aformention food into the opening at the top (to concentrate the food in the proximity of the sun coral and protect it from CUC and fish that may attempt to consume the food.
My sun corals had an excellent response to mysis shrimp and would fully bloom within a few moments of sensing it in the tank