Rimfire ignition requires two things, first, that the is priming compound where the firing pin strikes and second, that the primer compound be properly crushed.
Because of the way rimfire cases are primed, there is a possibility that the priming compound does not get sufficiently distributed all the way around the rim.
You never notice this unless random chance puts that empty spot in the rim under the firing pin. Bad round? yes, DUD? not if it fires when replaced in the chamber so the firing pin strikes a different spot.
Next point is crushing the primer, properly. And here's where things get.. interesting, and slightly different than centerfires. What appears to be a "light strike" can be the result of a normal strike (nothing wrong with firing pin strike itself) where other factors result in cushioning the firing pin blow.
The rimfire case needs to be completely and fully chambered, with the front face of the rim solidly contacting the barrel, so that the firing pin crushes the rim between the pin and the barrel, setting off the primer.
Crud build up in the chamber or the rear face of the barrel can prevent this. Enough build up (and being much softer than steel) acts like a cushion. The gun shuts normally, but the cushion of crud softens the blow enough so the round doesn't fire.
The firing pin strike on the case normally looks ok, making it difficult to determine the cause from the "dud round alone.
DIfferent brands of ammo have differing levels of "sensitivity" and different guns have different levels of power in their firing pin strikes.
Shooting with a friend one time, him using his S&W M41, me Ruger MK I, both shooting CCI Blazer 22LR. after half a dozen or so failures to fire in the first few mags, he passed me his unfired ammo and the "dud" rounds, and got different ammo from his bag, which ran flawlessly in his gun.
That CCI stuff he gave me ("duds included") ran flawless in my old Ruger, every round firing first time, including those that didn't fire in the S&W.
Guns with a long heavy hammer fall can hit hard enough that a "crud cushion" may be overcome and the round fires where the same situation in a gun with a "lighter" firing pin/striker could misfire.
So, if the firing pin hits, and the round doesn't fire, AND there is priming compound under the pin, its a "light strike". Why its a light strike could be a few diffrent reasons.
IF the round doesn't fire because there is no primer where the pin hits, THEN, it is a bad/defective round.