Exactly the same problem here. I screwed it back in once or twice but not it will no longer catch the threads at all.
I also thought the screw had somehow become to short or threads stripped ,
but in fact the screw is now too narrow to catch the threads on the mushroom valve holder.
Perhaps the valve holder cracked/expanded slightly similar to what Eddie described above - but either way im stuck and open to ideas.
@stephen how did you fix your problem?
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