Another white user very nice
What powder are you using, to me its one of two things possibly a stronger spring might be needed or it could be that the nipple is too fat. The solution is to slim down the nipple with a sharp file or replace the entire nipple-breechplug with a new one, same as used on all White rifles.
You could email or call Doc, I have his number if you need it PM me . I've called him plenty of times