[4 Scale]

I would suspect either the breechblock is binding, the mainspring is too weak, the firing pin/sear combination is somehow dragging or otherwise interfering with the breech bolt's forward travel, the mag is interfering or some combination of these.

To test the breech bolt for binding, remove the receiver from the frame, remove the toggle from the receiver, and remove the firing pin, spring and firing pin guide. Then slide the toggle into the receiver (with receiver off the pistol), work it throughout its travel to see if there is any binding. There will be a bit of resistance even with a Luger that is functioning properly, but it shouldn't be great. It is helpful to do this with a Luger known to cycle correctly so that you can compare resistance.

The only way I know to test the mainspring is to test fire with a stronger mainspring.

For firing pin/sear proper function, careful inspection and and perhaps careful smoothing with emery cloth of the offending areas if a problem is detected but I'm just guessing based on my knowledge of the mechanism, I've never had a situation where firing pin/sear issues cased the toggle not to close. I would be quite careful in removing material, as it is I suppose possible to make the firing pin release malfunction i.e. the the trigger pull is too light or the pistol fires when not intended, the nightmare of every home gunsmith. Anytime when messing with triggers and sears, consider that perhaps that is job for a genuine gunsmith.

To verify the mag is not the issue, I would test fire the pistol with no magazine, i.e. a single round in the chamber with no magazine in the pistol. The magazine follower button pushing against the hold open device is what causes the pistol to hold-open on last round; with no mag it should return to battery. If the toggle functions correctly with no magazine, but malfunctions with a magazine it, you've isolated the problem somewhat to either the magazine or something the magazine is touching.

My gut says it is the magazine. It is quite surprising how sensitive some Lugers are to which magazine is used. I had to try five magazines, two originals and three MecGars, until we found a magazine that would fix a failure to hold open issue (a different issue than you are dealing with, although similar in that the toggle was not functioning as it should).

Good luck and let us know how it goes.