Did you only try this once? Perhaps the bolt wasnt completely forward and the extractor never made it on to the rim. Manually cycling many weapons causes this scenario.
I though some would be curious about relative size. An interesting side note I believe the A2 stock and cheek piece way more than the entire lower with the Magpul ACS.
I bought a Stripped AR-15 Upper, BCM Gunfighter Tactical Charging Handle and a 16" M4 White Oak Armament barrel for my Colt Sporter project. This is on top of the Magpul ACS stock I ordered last week. I just need to decide on a flash hider and handguard.