They'll expand at all times of day, not just night. They're not filter feeders though - they want small meaty things like mysis, chopped clam, squid, etc.
When the tissue is expanded a little (not all sucked up hard against the skeleton) just put some food on it near it's mouth. The tentacles don't have to be extended. I think you'll see the food slowly migrate towards the mouth. That's the mucous coat transporting the food to the mouth. You can do this anytime the tissue is somewhat expanded. Like all LPS
though, they do go through periods of less activity than normal.
Have you got it in a low flow area on the sand? If not, maybe try a different placement?
I never had good luck with plate corals. I'm 0 for 3 on them. Mine were suicidal and moved themselves into positions where things fell off my rocks onto them. Then they could never recover from the tissue damage. They seem very hardy, but once injured they seem like they turn into quite a difficult coral to keep. At least that was my experience.
Sounds like you're doing everything you can... researching and babying it. Just keep trying to feed it now and then, and keep it blown clean if it's not expanding to clean itself off. That's all you can do!