your not letting your soil dry out enough, pluck any mushrooms in there out, remove the top inch of soil maybe even the top 2, depending on soil type and root size, if you see any roots stop right away and dont go any deeper, be very careful and remove small parts like archeologists do on digs so you know you dont go to deep, dont add more soil yet, wait till you've let the soil in there dry out pretty good, then add new soil. or just try letting your soil dry out better and the musrooms should all die, but since youve said they came back last time i dont know if it will be enough to stop em, cuz ive had this happen and so have others here and i just plucked em out and let the soil dry out pretty good and cut down on watering after that and they were gone and never came back.