Sounds like an ejection problem since the spent round was being extracted.
First of all, make sure the ejector is functioning properly. Take some lubricant (CLP type) and work the ejector. Clean the chamber good, and lubricate your bolt and carrier good. Lubrication is the key to a good functioning AR.
If the problem still occurs, replace ejector spring. You should have a spare parts kit for your gun, i.e., extra springs, ejectors, extractors, etc.
I had the same problem with a Colt M4 upper attached to my BM lower. Once I replaced the ejector spring it shot fine.