Should mention that I pulled a reload and a factory bullet when I got home and the factory round went off but the reload did not (just primer).
It has me stumped.
How much of a firing pin strike are you getting on the reloads? Are the primers going off or are you getting just a "click"?

FL sizing shouldn't be an issue if factory ammo is going boom. The factory ammo probably has a shorter head to datum length than your FL sized brass.

My first guess would be the CCI 400 primers. I use nothing but Rem. 7.5 BR in my AR.