Exchipy provide a good summary. When you go to pull the trigger, it may not move. There are a slew of reasons this will happen: the trigger didn't fully return for some reason (part of your glove is caught between frame & trigger/internal problem), a cratered primer has extruded into the firing pin hole, a swollen case is jammed against the breech face, etc. The sure cure-unless you've got a bullet stuck between cylinder and forcing cone-is to give a hefty yank on the hammer spur. That'll usually solve the problem. If not, what's your plan B?
When we were revolver armed, we shot 681's (a whole lot) with Federal 125 gr Magnum JHPs as the only ammo. Issues happened that our S&W Academy trained instructors never covered (cause revolvers always work). One example: during (no excuses except gun failure) qualification I had the extractor rod refuse to move and the speed loader already in the other hand. Fortunately, we were at the 25 yard barricade, so I whacked the rod against the barricade to clear the cylinder. Afterward, I found one empty with a FLAT primer and a firing pin indent you could barely see. If I hadn't cleared it, I'd have failed qualification. On duty, probably worse.