"I have one Lee mold that will fin unless I invert it and smack it with a wooden dowel every so often" what he said!
My NOE moulds tend to be stiff at closing due (I think) to the blunt alignment pins so the moulds do not close easily even when sprue plate lube is applied to the pins. I just give the edge of the handle a gentle tap as I close the mould and that does it.
Could be that when you are working with the mould it starts binding a bit on a pin so the blocks don't quite fully close. Try sprue plate lube on the alignment pins and if that doesn't do it then try a gentle tap just as you close the mould halves.
Longbow
PS: I love my NOE moulds so don't misunderstand. It is just a quirk I solved and continue to do. The moulds cast perfectly as long as they are closed.