Hi Guys,
the locking system relies on the sharpness on the edge of the hole in the plate, if you file across the plate,this will cure the problem, as already mentioned, if you turn the plate over this will solve the problem as well.
good luck, and don't fall in.
Brian.
ps .....only file across the flat of the plate, do not file into the hole,this done just to create the sharpness on the edge of the hole, don't over do it.