If you lightly tap the sprocket onto its taper, without the nut and washer, it should ‘pick up’ onto the taper, and be difficult to remove without a puller or a tap from the other side. If it won’t do that there is probably some damage to the taper on the mag or in the sprocket. If the sprocket is slipping there is very probably some damage to one of the tapers. If only slight you may be able to cure it by lapping the two items together with some fine grinding paste, but don’t overdo it, and do very carefully remove all traces of grinding paste before retesting the fit. Also make sure that there is no oil or grease on the tapers.