I agree with Lindsay. Take the rock out of the tank to work on it. I use a large bowl to hold it while it's out of the tank. Then remove the rock, cut the mushroom at teh base where it's attached. I use bridal veil to hold it against a piece of rock rubble or another small rock. I either rubberband the bridal veil or crazy glue it to the rock. All goes back into the tank. Place in an area of low flow. You can remove the bridal veil in about 2 weeks and the mushroom should have attached.
What kind of flow is it getting now? I had some shrooms
shrivel when a ph was moved and they got too much flow. Some detached and attached in another part of the tank. Some just shriveled up. Where once there was one, now there are several colonies.