[ithacaartist]

The hold-open's function is impelled by the mag's follower button, which rises under the pressure of the mag spring, to push up on the hold-open after the last cartridge has been stripped from the mag.

If you've already to see that the part, and its spring, are not bent, nor the edge of the part that is presented within the action to catch the block and hold things open, is chewed up, or worn to be rounded, this leaves only two things possibly at fault--the previous relationship, or a recoil spring that is a bit too strong.

If the mags all work in other Lugers, and their notches are relatively crisp and not worn, I'd think the mag is not the problem.There's an outside chance that the little ledge on the mag release button that pops into a mag's notch when inserted fully, is worn in a manner which would allow a mag to click in, but in a position that is a hair lower than what should be. But when these wear out and lose their sharp edge, the result is usually a failure of themag to stay installed.

If one were to change a 9mm pistol's barrel to a 7.65 mm without changing the main spring, the breech block might not be making it back far enough to allow the hold-open to pop up in front of it to block its return because the .30 Luger cartridge supplies insufficient energy to cycle the action all the way every time, due to too much spring pressure for the energy to overcome.

Try removing the grip panels (careful on the left one, to avoid chipping it near the safety lever), stick your finger up in there to make the hold-open rise, to see if its travel isn't impeded. If it is scratchy, or you suspect goop under it or its spring, remove and clean/smooth the area. If it works positively, that is good It might be that a 'Work-around" is necessary, which in this case would be a follower button that is a larger D. than the usual. The head of a larger D. button would touch the hold-open sooner and hold it a bit higher when the mag is securely in place.

You'll likely receive other suggestions, so collect a few more before deciding on a course of action.

(edit) p.s. Looks like John has hit on the likely explanation and remedy while I was typing all this. I think he has chosen wisely!