[4 Scale]

I replaced an original side plate on a Luger with an aftermarket side plate and experienced the exact same issue. The problem was, the transfer lever (aka "trigger lever") in the new side plate did not have an adequate bevel where it contacted the sear plunger. The bevel is necessary to enable the sear plunger to slide under the transfer lever during the reset process. Five minutes with a file and then polish with a sharpening stone fixed it.

This is one example of a malfunction in the area of "trigger and trigger lever engagement" description in post #4 above; other malfunctions could create the same issue.

To see if the issue is plunger reset, you might perform the following test preferably with dummy rounds (snap caps), although I suppose you could use live rounds at the range. Chamber the first round and pull the trigger, releasing the firing pin. Keep the trigger fully depressed as the next round is chambered. With the new round in the chamber, slowly release the trigger and you should hear a very light click as the sear plunger resets under the transfer lever. This should help isolate the problem location: no click=plunger reset is the issue, click=sear plunger reset is not the issue.