[DonVoigt]

That jam is typical of "weak" Fiocchi and the problem is that the toggle does not come back all the way to pick up the next round.
Solution is "stronger" ammo like PPU, or weaker mainspring.

You can't fix it with a magazine. Too short OAL can compound the problem, but unless you get the toggle to full extension, it still won't work.

I'll bet the toggle does not lock open when you fire a single round either, a confirming symptom.