Luger toggle problem

[PREDATARRRR]

How's it going everyone? I bought my first P08 a little over a year ago and it was actually my first gun. It's a 1916 DWM and I believe it's a German military model based on the proof markings. It has all matching serial numbers except for the magazine, and aside from some trigger lever issues that I managed to fix, it still fires great. My only problem with it is the toggle won't lock back after firing the last round in the magazine. I can manually lock it back with an empty magazine in, but it's only ever locked back once on its own after the last shot. I've fired 115 grain rounds in it as well as 124 grain rounds. Could this be because it doesn't have the original magazine? If so, any suggestions on how I can fix it? If not, any idea as to what the problem might be? Thanks in advance.