Assuming the primers did not ignite, there are two basic categories of cause:
- Cushioned firing pin blow
- Failure to get fully into battery so the firing pin blow is too far to the side.
The cushioned firing pin blow comes in one of three flavors:
- High primer — Primers should be seated 0.003"-0.005" below flush with the case head.
- Inadequate Crimp — If the crimp die didn't remove the flare from the expander die fully, the case will resist full entry into the chamber. This is what Mahavey was getting at, I expect.
- Seating out too far — If the bullet meets the throat of the gun before the case head is flush with the back of the barrel it can cushion the firing pin some as it is driven into the rifling; this is especially so with lead bullets.
The failure to go into battery fully also comes in three flavors:
- The gun doesn't feed your bullet shape well — this is usually due to feed ramp shape or sometimes where the magazine fingers release the bullet.
- The other two causes are the same as the last two for cushioned firing pin blow, just expressing themselves differently.
If your failures to fire don't include a way off-center firing pin strike, look to the first category.