Is his an older gun? It would have a different firing pin system then your newer 686+. Different strength springs can cause some differences in primer strikes between the two guns. Was he shooting DA or SA? When you fired the rounds that his gun didn't fire did you shoot them DA or SA?

I have a 686-3, at times shooting plated WC's if I don't run a crimp there are a few that don't easily drop in to my cylinder. The coated bullets are .358, plated bullets are .357 so some of the small differences and variations between the two guns can collide in results.

Also as mentioned might be some cylinder build up that isn't allowing the bullets to fully drop in. If running lighter loads they may be running a little dirty by not allowing full expansion in cylinder and getting some carbon sooting blowback in the cylinder chambers.
Karl