I have struggled with Duncan’s. Perhaps more than any other coral. It’s not that they die. I have had same issue as you. Sometimes they just don’t open. I had one closed for five weeks. I presently have three of them that I move around the tank. Funny thing is they don’t all like the same parameters or conditions. I have been unable to pinpoint if it’s flow,or lighting I have put them next to each other and in about five days. Only one opens Move them and a different one opens and the original closes. Presently I have two open and one closed. Closed one has been closed for three weeks. He will get moved until I find his happy place. Only trick is you need to leave them be for awhile once you move them. It takes time before the come out even in ideal situationshas anyone ever had any good luck with dunken coral. I never do. I have a very small colony and I can't get them to open at all. The frist 2 days after i got them they were fully open after that they closed up and have not reopen in 2 weeks now.