FWIW, every time I have had a primer fail to fire on the first attempt, and it did fire on the second attempt, it was a primer that wasn't adequately seated. The first attempt drives the primer to the bottom of the pocket but absorbs enough of the energy to fail to ignite the compound. The second attempt provides the energy to ignite the compound.
During seating the anvil compresses the primer compound when the primer is properly seated, thus increasing the sensitivity. And it should be below flush (0.002" to 0.005") when properly seated, but of course that all depends on the geometry of your primer pocket. If it is rounded at the bottom corner, the primer will contact before the anvil is properly seated.
I uniform all my primer pockets the first time through the brass, and you will not believe how "bad" some pockets are. So your brass might be part of the cause, especially if you had the same issue with Federal primers, which are known to be more sensitive. Just for info, I will include a picture of a primer pocket after I uniformed it.
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