If you're still having trouble getting it to attach try this.
Get a large frag plug that's larger than the mushroom.
Get a section of pvc pipe that is nearly the same diameter as the frag plug and cut a 1/4" or slightly thicker ring off. The thickness of the pvc ring depends on the mushroom.
Glue the pvc ring to the frag plug. Be sure to only use a tiny drop of super glue as the ring will be removed later on.
Get some white bridal veil material from Hobby Lobby. You want the super thin silk stuff if possible. If not the polyester one will do.
Place the mushroom on the frag plug and wrap the entire thing with the bridal veil and wrap a rubber band around the mounting stem of the plug to hold the material in place.
Place the secured frag plug where you want and keep an eye on it for a couple of weeks.
Once it attaches remove the netting and pvc ring from the plug. You can cut the stem off as well to mount the plug to the rock.