Thanks, ive already went thru the trouble of stripping the lock,looking for some stray plastic from the molding process,didnt find much.
oh well, my advice is dont do that as it is a p.i.t.a.
to reassemble.
nevertheless,i got the phenomena to repeat itself,but it seems a rare occurence:
if I insert the cartridge slowly,it is possible [with me skinny waif thumbs]to not push up the shell carrier to where it will stick. so it snaps back against my thumb before i get the rim past the mag stop.
Inserting the shells with more force works every time.