It is important for there to ample moisture for the worms to thrive but not too much for the bedding (hay, leaves, lawn clippings) to form an anaerobic environment. If there is not enough ventilation, and if your bin doesn't breathe well an anaerobic environment will develop. I find it is best to use a wooden bin because it breathes well, however plastic will work well too if there are enough holes drilled in it, and perhaps a fan blowing across the bin. I also like to fluff my bins, another way to achieve a good moisture level is to think like a wrung out sponge, your bedding should be that level of moisture. Moisture level being proper is critical for optimum breeding production.