Sounds like a bad piece of brass. I loaded up a bunch of 7 Rem Mag virgin brass casesanumber of years back and, as Murphy would have it, the gun misfired when aiming at an elk. Second shot went off and did its job. Later, I checked the dud round. Good primer dent. Pulled bullet - case full of powder. De-primed and discovered the case (Remington nickel)was made without a flash hole.

Since then, I run each piece of virgin brass through the NSO die to true up the neck; and to make sure the decap pin finds a flash hole in each case.