When my toggle rises up, it doesn't go back far enough to strip a round from the mag. I can still fire singles loaded into the barrel.

I have tried the original magazine with the gun, a Mec-Gar one, and a cheap copy version and none work.

I am guessing the main spring is too strong, but would like to hear some other ideas before I try to hand fit it.

I'm kinda new to the Luger but I'll try to help(?).

 1) Take it apart and CLEAN everything really good.
 2) If you are shooting factory loads you might want to buy a different mainspring. DO NOT CUT the original spring!
 3) If you reload you can work up loads like I did. Standard loads would not work the toggle, a little hotter loads would work the toggle but not lock the toggle in the up position after the last shot, then just a little hotter and everything worked fine. The guys at Simpsons LTD told me that 20% over standard should be enough and it was really close.
 4) Check out some real Luger forums for more info.
 5) There are a lot of different opinions on this subject. Read them all and try not to get too aggravated!

Mine is a 1939 Mauser mfg WWII issue.

As I've written many times here before, the Parabellum should work perfectly with very mild ammo if it's in proper working order. No need to use hot ammo.
I've had my 9mm pistol's recoil spring changed to a very, very stiff one and it still works fine with mild loads so it's hard to believe the spring would be the culprit in your case.

A thorough cleaning and a good oiling is the first thing to try.
Try to see/feel if something is binding somewhere. Does the lock slide in the receiver effortlessly (when you have stripped the gun and the recoil spring is not involved)?
Is it ejecting properly? Are you sure the lock doesn't go all the way back? Maybe it goes all the way back too fast and because of this the next round doesn't load properly and instead the lock gets stuck behind the cartridge in the mag?
Or maybe there is a problem with the way the magazine seats into the gun?

Probably the best thing would be to take it to a gunsmith who knows a lot about them.

In terms of internet assistance, you'll find the real experts here:
www.lugerforum.com/

It seems to be working now with 124gr Nato ammo. I would still like to find out why it doesn't work with 115.
Maybe the spring isn't strong enough? When it chambers a round sometimes it won't lock in fire posistion.
I have to manually pull back and push it down hard.

Quoted: Maybe the spring isn't strong enough? When it chambers a round sometimes it won't lock in fire posistion. I have to manually pull back and push it down hard.

How stiff does it feel when you pull the lock back?
Kind of hard to say what it should feel like, but you should have to "grip it properly" and use some force, like on any other 9mm semiauto pistol.

Sling-shotting a Luger toggle shouldn't take any more effort than any other semi slide. Yes, the toggle will need to be pushed down if the slide is let down softly.

As others suggested, clean the gun thoroughly & then oil the hell out of it. I shoot 115gr ammo all the time out of mine w/o any problems.

Make sure your newer mag isn't a USA brand, they're out there as well.

My .o2

It might also be a magazine spring issue. The faster 115gr is cycling too fast for the round to be stripped.
The 9x19 Parabelum was originally loaded with a slower burning powder than todays ammo.

I had a G date East German 08 a long time ago that never worked with modern loads.

Any clean & oiled Luger that's within specs will run ok with any current 115gr ammo, generally speaking.

124gr is ok but it's not required as a necessity.