Bullet seating causing problems?

[HUNTER2]

Double strike primers- was loading some 300 rds and had some rounds that took 2 strikes to fire. new brass - good and tight primers- primers replaced with hand tool with no damage- 180 Speer backwards- 165 forwards, 200 gr cast- tried different primers- once fired brass---still having one now and then. Then I noticed one with a level primer! When primed, they were all checked to make they were below level.. All primed pieces checked ok....Tried loading some more....found another one....these are not compressed loads....Am I compressing the air in the case so fast it is moving the primer enough to equalize pressure? Loaded up another 100 seating the bullet slowly with no problem! Now trying the other variables with the slow seating and so far ok. Had been blaming it on a weak spring of questionable primers.. I load for 357 up - this is a new one on me!!! Anybody else?