Help with hold open device
 

I am trying to repair a luger and the hold open device will not work. It will hold the toggle back if pressure is applied to the bottom of the magazine. If the mag is pulled even slightly, the toggle slams shut. I have checked the hold open spring and part and they seem ok. I have tried other magazines and while they work fine in other lugers I have, they won't work in this gun.
The gun has all mismatched parts and has been refinished. Perhaps this has caused the problem. Any help here is greatly appreciated.

Magazine Button latch is worn. Since the gun is refinished, get a replacement from LugerDoc

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!

I appreciate all the advice. It does look like the mag catch could be if not all at least part of the problem here. The gun is 30 cal. but I have not fired it. I only tried to get the hold open to work manually. It is possible to pull the mag free without pressing the mag release button. And then the toggle will slam forward. I have found no matching parts anywhere in this gun. And some are obviously aftermarket. The sideplate is aftermarket and so oversized that the gun was almost impossible to disassemble and the takedown latch was jammed. A bit of careful filing and polishing fixed that. Thanks again for the help.

Update: The gun is fixed and working well. I did take into consideration all the suggestions. It seems that the aftermarket holdopen was a bit oversized and dragging. After a bit of polishing the holdopen and tweaking the spring all is well. Thanks again for the help. It always feels good after a successful repair.

Did this fix the issue with the mag that's right on the edge of staying in/dropping out?

Yep. All seems ok now. I will further test.