Short barreled uppers can be picky, especially 7.5" uppers. They take a bit of punishment. There are a few things you could do to try and fix the problem.
First: I would do an extractor upgrade on your bolt. You can buy the O-ring upgrade kits from several online companies. I would suggest you try Bravo Company.
Second: What weight buffer do you have? If you are running a standard weight buffer, I would suggest you go with a heavier H or H2 buffer. The heavier buffer will slow down the cycling, and allow some of the gas pressures to subside before extraction begins. Switching to a F/A bolt carrier will also accomplish the same thing, but switching buffers is much cheaper.
I am betting the O-ring extractor upgrade will solve the problem, but a heavier buffer may also be needed.