This happened to me when I first started casing, but it is hard to actually pin down the problem without more information.I have come to the realizaiton that I was overwatering my casing. The rule I use is, once pinning begins, let them be, the shrooms have made the biological choice that enough moisture is present to sustain fruits.
This is only a guess of course, it could be a number of things, maybe more info about your setup would be helpful, happy growing.
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Sometimes you get shown the light in the strangest of places if you look at it right.