Because the mech is rubbish
In my case and I suspect many others
The idea for a leaf switch to be activated by a post and spoon was meant for pops and has proven effective.
Stern engineers took a cheapskate solution for IMDN by copying a design intended for pops.
in IMDN the ball, rod, spring, pencil post can bind and get sticky. Even after complete disassembly clean and lube it can still bind.
the spoon leaf sw tolerance are to tight.
even if you get it adjusted okay the leaf is fine and flimsy can cause bounce and false hits.
Overall the mech is junk and needs to be replaced with something reliable.