I don't understand these things either. I had one that was spectacular - about 12" across when open. It slowly started to recede from both edges until there was only a tiny (about 1" spot) with any life left on it - about 3 bubbles.

I was about to dispose of it, but decided to stick it on a small shelf in a non-visible spot until it died completely. Now, about 3 weeks later, it's recovering very well - it has reclaimed about 50% of it's skeleton and visibly growing every day. Sure happy I didn't toss it.

I have also seen a similar behavior from one at our LFS - is it possible this is abit of a cycle they go through?