Sounds like you're holding too much moisture in your soil, especially with such small drain holes, if the soil is resting right on the drain holes.
You could unpot the plant, wrap the soil/roots in newspaper for an hour to absorb excess water, widen up the drain holes, fill the bottom of the bucket with large gravel or broken terra cotta pottery, then replant and stir up the top inch or two of soil. You can also put a small fan aimed at the top of the soil and a fluoro or incandescent light close to soil top to dry it out faster.
I only had 'shrooms once, on one plant, and I found the drain holes clogged and that I was holding water, too. That's what I did, and it worked fine.
Now I'm feeling guilty for wasting those shrooms. I don't know why I didn't think to chop 'em up and let 'em just compost there in the soil.