That helps.
it's a seating issue of the primer in the breech pocket.
You need a shim, a different breech plug or try a different brand of primer.

And the reason you were asked what type of powder you were using.
You need a different flame channel in the breech plug to ignite BH209.

Been down this road myself.

Forgot to add. Deep clean the firing pin. If you have blow back that could be dirty or damaged?

JW