My trick is to make sure I know which side the thing should seat on -
hint - the plate is rounded on one end and square on the other - one end holds better in the slot - should match the roundness so it 'seats' right.
Secondly - I hold the mold so I know which side is 'bottom' - and as I put it in the vice, I tap it so that I am sure it's seated - look thru the injection port - you can tell if it's right - just before I apply the pressure with the vice.
(IOW - put the plate in - hold up the mold 'bottom down' so it is in the slot - hold tight - clamp.