Ive been keeping a couple colonies of sun corals for a while now and they are one of my favorites too. I had a similar issue shortly after I got my first colony. heres a few things to keep in mind that might help you or someone else with these corals
They are nonphotosynthetic meaning they dont need light at all. This being said they can have a problem with to much light. Always start them off in a low light area. they do fine in areas that dont get direct light like caves or under over hanging rocks. you can aclimate them to stronger light just do it slowly and they should be fine. I have one of my colonies right next to a sps colony and they are both doing great.
I find my colonies do best in low to medium flow areas and can do well in high flow areas its just a bit harder to make sure they get fed with the higher flow. just dont put them in no flow areas.
keep the colonies out of the sand bed if you can. If you get sand or other debris between the polyps the tissue with die off under the debris pretty fast. its best to blow the colony clean on a regular basis to prevent this.
feeding is the most important with the corals. being that they are nonphotosynthetic they get all of their energy from whatever they get fed. these corals have a huge appitite and will usually eat way more than you think they can. you really have to spot feed these corals. general tank feedings wont be enough for these corals. I use a turkey baster to feed my polyps a mix of mysis and brine every day to every other day at the longest. one colony about 4 inches across can consume atleast one cube of frozen mysis a day. a good way to tell if you're feeing enough is to look at the polyps when they are closed, if they look sunken down into the skeleton then they need to be fed. if they are swollen up over the end of their skeleton then they have enough.
they are very hardy corals if they get fed properly, you can even train them to open during the day for feedings.
Hope this helps