I have a Colt Mustang that I obtained recently (sort of-long story), it had been taken apart and reassembled improperly. I had a gunsmith do some work on it but don't think he took removed the extractor mechanism.

I bought a box of 50 rounds of some kind of ball ammo-White Box maybe (but I don't remember & I threw the box out) & a box of Winchester 85 grain silvertip hp-since I would guess that this is the best defense round in this caliber.

I ran the box of ball through it and about 25 rounds of the Winchester Silvertip through it. The gun functioned without a hitch. I cleaned the gun after firing it. I'm good about cleaning up guns and I made sure I cleaned this one because I figured it might be more sensitive to cleanings than my Glocks and Beretta 92.

I bought 3 boxes of Remington UMC 95 grain ball ammo- the box actually says 95 gr mc L380AP on it.  Today, I think in the second mag through the gun, a spent brass failed to eject. I think this was on the third shot. I cleared the malfunction and I think it might have fired the next two rounds. After that it would have the same malfunction on every round.

The malfunction was: the pistol would fire and the slide would come back a little, enough that I could see the brass in the pistol, but it would not go all the way back and eject the brass and load the next round in. When I then manually operated the slide the pistol would eject the spent brass and load the next round.

I had 10 rounds of the Silvertip with me. The guns fired the two magazines of Silvertip with no problem. I only have these two magazines, which are the original Colt magazines the guns came with as far as I know.

I took the pistol apart when I got home and although it's dirty from being fired, nothing really looks amiss to my untrained eye.

Any ideas as to what's up? Is this gun just a little finicky about ammo selection?

Quoted: could the rounds you bought not have enough power to cycle the slide??

They are standard factory Remington UMC rounds, the ones that come in the yellow box. I have never had a problem with this ammo in any other caliber or gun, but I don't know-I don't know anyone else who has a .380 off the top off my head that I could check them in.