[John Sabato]

Welcome to the Lugerforum TW.

Since you have described the gun as a mismatch... it is possible that the original recoil spring is inadequate. It could also be possible that the "S" link that pulls the toggle back down after recoil may be bent out of specifications and allows too much travel.

But before we get into mechanicals, what ammo are you using? it may just be to hot a load for your gun. Winchester bulk 9mm ammo sold at Walmart gets good reports on this forum for dependability and function in most Lugers. You might try a box of that...

IS the rear toggle pin serial numbered to the toggle and upper receiver? If it is too loose a fit, it could be contributing to the problem by movement during recoil. A slightly tighter fit of this pin might help things too.

Ideally, the upper receiver and toggle should move sufficiently on recoil to activate the holdopen if the gun is loaded with a single round in the magazine but with sufficient spring tension to prevent the frame from taking a pounding or allowing the rear pin to do what you are describing.

I would start with a new recoil spring, then if that doesn't work, try replacing the "S" link, and finally try a new toggle pin that fits tighter...