The magazines that came with the postwar Mauser Parabellum were not very good. Try using a MecGar as an alternative magazine. I never got my Mauser Parabellum magazines to function at all and basically gave up on them
There is quite a lot of reference to the lack of quality of early Magazines in the Mauser archives of the 1970s. The magazines were produced by a Dutch company that had quite some trouble getting things correct.
When Mauser tested several ammunition brands around 1970-71 they found that Winchester performed well, but that the toggle would not always stay open after the last shot. I've observed the same with the Winchester 115gr 9x19 and a 1937 Mauser. Wouldn't worry about it.