My plate is doing great under PC lighting with lots of direct feedings (every other day). Is yours a short or long tentacle plate? The reason I ask is that the long tentacle plates I hear are much harder to keep.
What is yours looking like and how long have you had it? If it simply isn't inflated, I wouldn't worry too much, as they usually aren't inflated at all during the day in their natural home--only at night.
Is it in a lot of flow? This can cause problems with some plates, just like other LPS whose flesh can be easily damaged against their sharp skeletons if they are in direct current.
If its tissue is receding from the edges it can be bad as they tend not to be able to outcompete algae if it takes hold on their skeleton. However, if it starts to recover, you could get new polyps sprouting where the tissue receded and end up with babies.
I might be cautious though, about moving it to a completely new tank, as the stress of the move might decrease it's chances of survival. Can you take some pictures and give a more detailed description? Maybe all hope is not lost...
Any problem with your water parameters?
In general, they should be kept on the substrate, out of direct current. Many say they do not need much food other than light but I tend to feed as much as it will eat (why would it have such a large mouth if it didn't like to eat a lot?). As for lighting, I have mine under 80 watts of 50/50 lighting in a 10 gallon tank, and the half of it that wasn't shaded bleached for about a month but is now back to normal (I don't think they need tons of light).
