I fixed an Erfurt Luger that was doing this yesterday. It needed a new spring for the sear bar plunger button and the trigger bar (part in the sideplate) ground down due to over rotation of the arm. Hugh has a good method of fixing this problem listed at the top of this section of the forum listed as "cocking problems" as a sticky message. Tom Heller (Herr Luger Doktor) supplied the parts.