Mine never goes dormant. It has days when it looks particularly large compared to others, but its always open everyday since I have had it. it never just closes up completely and sits there as a bare skeleton. The corals that tend to just shutdown for extends periods like you describe remind me more of a Toadstool leather. That coral will just close up and won't show any polyp extension for an extended period then come back in full force. This is a normal behavior for them but not IMO for a LT plate.
Is yours on the sand bed? If it is, has sand been able to sit on the tissue and cover it up once in a while. That can irritate the coral and cause it to shutdown. Also Mine does not get too much light either. Its pretty well shaded.