When you fill the cup with substrate, it is better if you either have a good amount of substrate in the cup or put some sand in and then rubble on top. What I do is just make sure there is enough substrate to cover the whole bottom of the cup. Another thing, you could take a piece of rock, place the mushroom on the rock, take bridal veil and place it around the whole thing and tie with a plastic zip tie so that the mushroom cant wiggle around and put it in a low flow area of the tank.