Usually they just attach on the rock right next to the original, but the one that I thought was dying just fell off. I put it on top of another rock in the tank, and it attached to that within a couple of days.
Just read your other thread about the problems you are currently having in your tank, and unfortunately I'm now thinking that could be the issue. Shrooms are pretty hardy, as corals go, but they still need water quality at least good enough to support the fish. And if the fish aren't doing well . . . My understanding is that corals can only tolerate nitrates up to about 10 ppm (depending on the species) and no ammonia at all.