Based on some quick research, it also could have something to do with the sideplate itself as that is what operates the sear connection from the trigger on the Luger, IIRC. If you get the chance, try shooting it again and apply some pressure to the sideplate after you fire the first round and see if the problem persists. It could be that is what is needed to release the sear bar. Best way to fix the issue would probably be to try and find another sideplate altogether.
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-QM
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