I'm guessing your grease cookie lube is too hard, trapping air behind it.  And you're using the PP bullet to seat the cookie.  The lube should be quite soft like room temp butter or even softer.  Once you've punched out the lube with the case, top it with the wad & push the cookie down with a dowel, not the bullet.  Any air should be forced out around the soft cookie. Then insert the PP bullet.

Wayne

Lube is room temperature SPG, quite soft. After its in the case mouth I top it with wad and push them down together onto over powder wad.

I am using a Duplex load and lube as the rifle is a Pacific meant for hunting and I wish to avoid wiping between shots. 

If my puncturing method is how others do it then I will keep on.

I have that “issue” seating grease groove bullets in fired but unsized cases.  My fix is to put the cartridges in MTM cases, bullet down, with a pad of that black artificial spongey stuff that they use to pack electronics for shipping.  It needs to be thick enough to push the cases over the bullets until there is no air space possible, while still allowing the lid to snap closed.

The running fit of the GG bullets has enough friction to keep them from coming back out unless the cartridges are shaken. Add the sponge, close the lid, and in a few minutes the trapped air will leak out and whatever has been moving will stay put.

A paper patched bullet might need a little mouth reduction on an unsized case for the same friction, but the same principle applies.  By the time you open the box at the range, the bullets should be seated down with nothing moving.