Have you checked gas block alignment? Where does the gas tube end in the receiver, the middle of the cam pin cutout? What do you have for a buffer system? Buffer weight and bad springs play a role in many that don't run. You might want to measure the gas port size when you check gas block alignment, could be a small port. Someone should be along shortly with a more defined approach, search out the "waterboarding" thread for a list of required details for that troubleshooting process.