My .223 did the same thing with cheap ammo. If you're reloading...that's probably not the case with you. Once I switched to Black Hills ammo, I haven't had a problem in 500 rounds.

I found that if I wiped the brass of the cheap ammo with a rag that had some gun oil on it (just a tad) they didn't stick.

I think it's a combination of brass expansion and a poor system of gripping the shell upon extraction.