When the bolt fails to lock up, the AR stops firing by design. The tell tale sign is the light firing pin strike on the unfired cartridge caused by the disconnect, correctly, capturing the hammer before it can fully strike the firing pin. If you have a failure to fire, with the dimple, look for why the bolt did not lock up, first. The hammer and disconnect are working correctly, so you have bad ammo or crud in the chamber and/or bolt recess, clean the gun with an AR chamber brush and check the ammo for bulges, bends, length. Just pull the take down pin and pull the bolt, clean, inspect, clean again.
The AR-15 must be kept clean, it craps where it eats, feed it good stuff!