I just purchased a colt Series 80- Govt Model MK IV in 380. It came with the original and an aftermarket clip, and two boxes of ammo. Magtech is crap and I expected it. but I also tried two other boxes of jacketed ammo, one was UMC and other is Remington. Can't get though a full mag without a failure to eject. The spent round is pulled about 1/2 way out, and the slide jams on the next round in the mag. (Note: I have not had a stovepipe or FTF)
I don't know if the springs are original or not. the gun is in good shape but obviously carried a bit. Should I put a fresh, original spring kit in it? Increased power, reduced power? I have thoughly cleaned it and applied a small amount of lubriplate to the slide. But it doesn't help.
Can anyone tell me the probable cause.

May be your extractor is worn or dirty. Remove it and clean it inside and out.
If that isn’t the fix then replace your springs and or buy a new mag.
Other than that send it to Colt to have them fix it. They will.

I have a Govt model .380, I don't know about the Series 80 though. When it was new, 1989 or so, while firing my first 2 clips of ammo I had 2 missfires. I just re-cocked and pulled the trigger again and the ammo fired. I also had a couple of incomplete ejections. I decided that is was because my wrist was a bit too relaxed while I shooting it. When I realized this I just held the gun a bit firmer when I pulled the trigger and have not had the problem again.