I suspect your problem is your buffer tube/buffer/spring combo, with a AR15 type carbine buffer tube you have to use a really short buffer (around 2.5")
and a special spring with a higher spring rate that has a wider space between coils - the preferred combo is the CAR-10/Special AR10 spring from Slash:
http://heavybuffers.com/ar10carbine.html
If you don't have the dough for that, the standard LR-308 Carbine Buffer/Spring from DPMS seems to have the correct dimensions, I haven't fired mine yet but
the measurements work out correct for proper cycling:
http://www.midwayusa.com/product/232006/dpms-buffer-lr-308-carbine
http://www.midwayusa.com/product/813595/dpms-buffer-spring-lr-308-carbine