Other than the shooter....
Ammo is probably the most "General" issue with centerfire semis. For some guns (those with either old worn out or plain cheap mags available) mags are the most common issue. But that's typically easily solved by using quality well maintained mags. The only gun I've had mag issues that I've owned was a officer's frame 1911 that had mystery mags with it when I purchased it. I bought quality mags, and voila! it was cured. But I've had ammo issues with my Glock (didn't like light loads I was doing for my range toy 9mm), blunt hollow points not feeding in the .45, bad primers, a backwards bullet in commercial reloads, etc.
Curious: Were these ammo related issues all reloads or did you experience the same thing with factory ammo too?