I was testing some ammo tonight when two round, consecutive misfired.
After clearing them afterwards the primers looked lightly struck, so I tried them again and they fired.
My first thoughts were that the primers were bad, as they are 15 years old and they have been through a flood. But on inspection of them after firing, they showed a completely different indentation. So I tried some Fiocchi factory ammo, and they show the same marks. It turns out I had never noticed that is what Glock firing pins are supposed to leave, but why didn't they fire the first time?

Can anyone suggest what might be going on?

upload img

Yes, Glock firing pin tips ain't round. If your pistol fires other ammo, then the primers are too hard. If it fired them before, then your springs have weakened. Or your primers got wet. Who knows. Don't use this ammo for serious purposes.

Were these rounds reloaded? If so were they trimmed too short? That could have caused the round to be driven forward with a light strike.

Clean the brass shavings out of the firing pin hole. Once every several gazillion rounds you need to do this, especially on older pistols that have non-chamfered firing pin holes.

Remove the firing pin assembly to do that. If you don't know how, find out how first so you don't mess up the plastic parts.

Usually, this is a sign they weren't seated deep enough and the first strike seated them properly. Second hit they go off, since they are now seated so they can absorb the full force of the firing pin.
Very common problem since primers were invented.
Run your finger over the case head/primer of all rounds after you put them in an ammo box and feel for raised primers.