When primers are assembled, the anvil is not set into the priming compound. This is in order to make them safe to ship and handle. When you seat a primer, you are not just inserting it into the primer pocket. You are also "arming" the primer by forcing the anvil into the priming compound, so that the firing pin/striker impact reliably ignites it. The normal specs for seating a primer are for them to be at least .002" below flush, up to .009" below flush. Too high, and the primer may not go off-too much energy is absorbed in seating the primer deeper. Too deep, and it is possible to crack the priming compound, resulting in the primer not igniting at all.
If primers fail to go off after the initial strike, but go off after a subsequent strike, it is most likely that primers are not fully seated.
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