Maybe the gas tube is not lined-up with the receiver correctly. Also...make sure the two gas key bolts on the bolt carrier are tight an staked in.

Do a AR check function with the gun unloaded an magazine not inserted --- Pull the charging handle back to cock it. Let the BCG (bolt carrier group) go into battery. Put the safety selector on safe --- Press the trigger back --- It should not let the hammer fall --- Put the safety in the off position and press the trigger --- The hammer should fall --- Keep your finger pressed back on the trigger and charge with the charging handle and let it go back into battery --- Slowly release the trigger while listening for the disconnector "click." You should hear the click --- if not --- it might be something wrong with the trigger group.