This might be the case for me as well. My tank is almost 2 years old, but I had to fight disease with reef safe products, that might have had a durable impact on the microfauna.I have had a very similar experience as yours. I've looked over all of the "reasons" people have given to cause this and same as yourself none apply. The general consensus seems to be that my 1yo tank, started with dry rock and sand, just is not mature and stabile enough for stony corals. Yes, I do have coralline algae growth but that doesn't seem to be the definite indication the tank is ready for more difficult specimens.
I guess only time will tell.