You didn't say if there's a dent in the primer when it doesn't fire?
Does it fire on the second try?
SA or DA?
Handloads or factory ammo?
Recoil from magnum loads invariably loosens screws that aren't fully tightened down. As
yfdcap mentioned, check the strain screw under the grip. Might as well check all the others while you're at it.
If that's tight you might have a firing pin that's of marginal length (.485-.490") & a longer one could be the answer.
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