The priming compound is a bit like the white tip of a "strike anywhere" match--in performance, not chemistry.
The small priming pellet is sandwiched between the primer cup and the anvil--built into the case if that S&B is Berdan primed, or pressed into the primer cup if the ammo is good ol' American Boxer primed*.
A skimpy dose of priming compound can result in a thin pellet that's not really sandwiched tight and which may even have a gap. The first firing pin strike can close up the gap. If that action neither crumbles the pellet, nor ignites it, you can have a second-strike success in firing the round. Primers not seated all the way down can contribute to this, as the firing pin strike completes the seating operation and soaks up energy that would have smashed the pellet and started the party rolling.
With many hammer-fired weapons, these primer problems can be imitated by the slide or bolt being as little as 1/16 back from fully forward. The hammer strikes the lower rear corner of the slide or bolt first--the angle if impact with back of the slide or bolt slows down the hammer before it strikes the firing pin. That same early impact almost always pushes the slide or bolt forward, making it look like nothing was wrong with the operating cycle.
*But wasn't Berdan an American and Boxer a European of some sort? Strange things happen.