[DonVoigt]

Hold open problems can be a pain.
One of my shooters is a 1906(coil mainspring/grip safety). I have two uppers for it.
One is a 9mm 1902 fat barrel profile clone; it holds open every time with any 9mm ammo I feed it.
The "other" barrel is a 4" bull barrel GT special, I was firing it today - used 4 different types 9mm ammo, two factory and two reloads. Only one of the reloads would lock the toggle open when firing.

So what you say has this to do with the OP's 7,65mm- nothing- but it does illustrate the need to get full recoil for the hold open to function! The slightly heavier 4" bull barrel is right on the edge of allowing full recoil due to the "extra" mass.

So- I'm back to "it's the ammo", you gotta' get some PPU!