[Ron Wood]

If the gun will eventually fire after repeated trigger pulls, I suspect that it has a "sluggish" disconnector on the sear (the little plunger on the front of the sear bar). When the toggle closes, stripping off the next round from the magazine and cocking the firing pin, as the receiver moves forward the disconnector hits the trigger transfer lever inside the side plate because you haven't had time to release the trigger. When you release the trigger for the next shot and the transfer lever moves out of the way, the disconnector is supposed to extend from the front of the sear bar so that it may again be depressed by the transfer lever to move the sear and release the firing pin. If it doesn't extend, there is nothing for the transfer lever to press on and it won't fire until the disconnector finally makes up its mind to come out and play.

Soak the disconnector with a solvent (WD-40 or something like that) and keep depressing it until the gunk is cleared and it works freely. It doesn't hurt to clean up the crud under the sear bar as well, as Jerry suggested.