mmm, yeah I'm not in front of my daw right now, and not ehm exactly clearheaded because it is 5 AM here.
(so late! I'm off to bed)
basically if you play a note it uses voices. memory allocations. too little ram and the stuff goes crazy.
but since a drumkit doesn't require much memory, I guess it is that priority note thing. it will for instance cut off an open hihat, when you play a closed one. one note overrules the other. this can also be the last note overruling the previous one etc. sorry to say, but you have to check your manual. it's in there somewhere