1911 malfunction question

Lately I've been having the same occasional malfunction, two or three per hundred rounds, in my Colt wadgun. What happens is this: the gun fires and ejects the fired shell, but the slide returns only about half-way with the next round partially in the chamber. When I lock the slide back and remove the magazine, the round that's part-way into the chamber stays where it is (doesn't eject). As I'm removing the magazine, the top round in it will pop out. The load I'm using is a 200 grain Magnus SWC and 4.0 gr of old, cardboard cannister, Bullseye. Slide-mounted Vortex red dot, 12lb recoil spring. This usually happens with the first or second round from a full magazine.

I tested the extractor for tension; it wouldn't hold a loaded round securely, so I have increased that, but the problem persists. I'll try different magazines next. But in the mean time, is there anything obvious I'm overlooking?

I have had same on mine 9mm. Problem was that the slide did not have enough "slap" travel to force chamber next round. This happened if 
1) recoil spring too weak and does not have enough force to carry next round in
2) combination of mainspring and recoil spring too strong, does not let slide far enough for a strong return to cary next round in. Solved with weaker 10lb recoil spring on 9mm, and 15lb mainspring and 7lb recoil spring on AA 22lr conversion. 

This is relevant specifically to 1911.

I had a similar problem with an old GoldCup using factory Winchester SWC.  The fix was to increase crimp on the factory rounds to ease the case mouth from biting in inside at the top of the chamber.  Might have been other problems too.  But this worked.

Thanks for the suggestions. I upped the recoil spring by a couple of pounds and, same load, same magazines, it just went 100 rounds with no malfunctions at all. Will see if this fix holds long-term.