I have run into this very often and one of the things that REALLY helps is a heavier extractor spring, either from Wolfe or the mil-spec "black" spring and buffer. Also with short barreled AR's you MUST keep your chamber very clean. Any gunk build-up will cause the empty case to want to stick and slow down the cycle process. A short barreled system is very finicky and must be kept clean. We have had limited success by opening up the gas port, they are usually already pretty large and I would not go beyond .090" as you will then over power the cyclic which can lead to other problems and you cannot re-drill the hole smaller. Since you state that you have eject problems and not extract, you can also check your ejector spring and try clipping a couple of coils if you have extra ones. Another trick we have tried with limited success is to place a small ball bearing behind the ejector spring to make it stiffer. But I would go with lightly polishing your chamber and increased extractor spring/buffer first.