I have a few ideas
1) do you shoot alot of steel cased ammo out of the gun ? If so do you clean the chamber really well after ? As I'm sure you know steel cased ammo some times does not seal the chamber completely and you can get blow back and build up in the chamber . When the softer brass expands in the worm dirty-ish chamber the case can stick . This it what helps contribute to wearing of or broken extractors .
2) Bad extractor or spring
3) how did the cases look . I'm just getting in to reloading and I remember reading something about if the neck of the case expands to much it can stick to the (something ) I don't remember I want to say rifling but that does not sound right but there was something about the case expanding to much and sticking .