Hang fires can happen occasionally, which is exactly why all shooters are (should be) trained to hold on aim and NOT OPEN THE BREECH after the click - timing varies from 10 to 30 seconds depending on whom is talking. Never had a hang fire take even close to 10 seconds - in my experience with metallic carts (rimfire and centrefire) the delay has been only a little more than long enough to notice, so well less than one second. I suspect that the 30 second timing comes from times gone by when mercury fulminate primers were the norm, but this is pure speculation on my part.
Usually a one-off and I have seen no learned discussion as to why they happen in metallic cartridge guns. If your reloads give repeatable hang fires then someone might be interested in doing some research on them.
Hang fires in muzzle loaders are much more common. In percussion guns usually attributed to blocked nipple, and in flintlocks usually attributed to over full or blocked vent hole (and in matchlocks hang fires are pretty much a design feature!).