TO begin with, verify that the snap caps that feed are dimensionally the same as the ammo (which doesn't).
Snap caps are close to live ammo size but not guaranteed to be the same exact size, as they are snap caps, and not dummy rounds.
Also consider this, the WEIGHT of a snap cap is not the same as a live round.
I ran across a 1911 that would feed snap caps, hollow points and ball ammo. It would eject fired cases, snap caps and loaded hollow points but would NOT eject loaded ball ammo. Hollow points ejected ok because they were shorter than loaded ball ammo. I found it interesting that snap caps, the same size as ball ammo would eject just fine, but ACTUAL ball ammo would not eject and would jam in the gun requiring about 3 hands and a screwdriver to clear the round as the slide could not be locked back or barely moved at all once jammed.
It was the weight of the loaded round that when ejection was attempted, gave it a very slightly different angle of travel than the snap cap, and that angle was enough different that the round did not clear the slide and jammed the gun,
Turned out the gun had the wrong ejector installed. Not your issue but I mention it to make the point that sometimes snap caps will show proper function when live ammo won't.