I think the same thing as Bigedp51.

The Lapua brass is the problem. It's very obvious after farther evidence provided by tizzy.

A very good example tizzy posted is the big gap in velocity from virgin to once fired.

The reason why the Virgin brass made so much less velocity than the once fired is because when the brass stretches this much during firing it delays the pressure buildup / peak pressure.

You can still save the Virgin brass by either making a false shoulder on the neck or jamming bullets hard in the rifling. The false shoulder on the neck is the best solution, but you'll need a bigger expender or bushing in the sizing die. Research "false shoulder reloading brass" on google and you'll save the rest of the good virgin brass.

On the other hand... I would first call Lapua and let them know what's happening. I'm sure they would want to know.