how did you acclimate it? I usually take about 2 hours getting all livestock used to the tank water where they're going. I the item is in a small bag, I roll down the bag a bit, secure it to the tank at the top with a floating magnet cleaner, then add about 1/4 cup of tank water every 15 minutes. When the bag gets full, empty 1/2 out & start again till it gets full. Larger bags of critters usually get placed in a clean bucket with their water, then I set up a drip system with airline tubing and adjust to 1 to 2 drops per second by tying a knot in the airline, or by using a plastic air manifold. double or triple the water volume oner a couple of hours.
A new mushroom should adapt to your water & lights within a couple of days. They seem to prefer lower levels of lighting, and if you have MH lights, the bottom is the right place. If you have VHO's or PC's your options are much more open as to placement. I have 4 110 watt VHO's over my 90 gallon, and have mushrooms growing everywhere!