Yep, What Roberts said.
In the carbine length gas systems the extractors certainly are more likely to lift off the case due to the higher operating pressures of the short gas tubes. The operating parts are under more violent force than the full-length rifle gas systems, therefore they need the increased ex spring tension in order to hold on to the case head/ex groove more forcefully. When I purchased my Bushmaster Heavy bbl 16" carbine, I immediately placed an "O" ring and heavy-duty ex spring, but in my Altamont Tactical Outfitters' custom 20" A4 rifle, I only have the heavy-duty ex spring on the FN extractor & bolt and have found it works great.
Also, take a good, close look at your bolt carrier and insure the gas key is mounted properly, ie; flush to the carrier so that it has a good seal and make sure it's properly staked to the carrier also.