2 strikes
Some primers are harder than others. Some seem to be more sensitive (don't require as hard/deep strike) than others. There can also be problems with primers that aren't seated firmly to the bottom of the primer pocket.
My father has an S&W Model 10 with a 2 inch barrel. He thinks it is the sweetest thing since cream from Jerseys fed on peanuts. Part of his infatuation is due to the buttery smooth double action pull - which is also relatively light.
My father is almost 81 and this gun has been a favorite of his for more years than I would care to count. He doesn't get to shoot it much - although the frequency has increased since I moved back in the neighborhood. Any way. we were plinking around the other day and had a bag full of old (really old) wadcutter & semi-wadcutter reloads, some old factory round and other similar stuff that we were burning up. He had his Model 10 and I had a Model 36 and a Colt Diamondback.
He had a number of failures to fire (maybe as many as 10) out of about 120 to 150 rounds. Some of these were failure even after multiple hammer strikes. I had none with either of my two guns. I took his failures to fire and ran them through one or the other of my guns with 100% function. Of course neither of my guns were as "light and buttery smooth" as the Model 10. When he was off attending to a call of nature I checked the mainspring tension screw. I could move it with my thumbnail. I didn't have a proper screwdriver, but I did tighten the screw some with the blunted end of the fingernail file blade on a swiss army knife.
I'm really going to have to "borrow" that gun and get some serious tension on the mainspring. Of course the trigger pull will be just as smooth as before - but not quite as light. Which I'm hoping he won't notice - but that's a faint hope. In the meantime I'm taking note of what ammo works reliably in his gun and trying to make sure that that is all that he has either in the gun or around the house.
As Mr. Sam says, it may be a light strike or a bad primer, but it needs to be looked into. Keep in mind that it may actually be both. Either, or both, need to be looked into.
Good luck.