Ditto with this if you are cleaning the upper receiver correctly (even before the rifle was shot), including using a chamber brush with CLP to clean the chamber, and cleaning and lubing the rest of the rifle with CLP as well  (maybe Sweets on the actual barrel bore, but keep Hoppes away from the rifle).

And again, the rifle has to be fully cleaned to remove the old asssembly/storge grease that can attract debries, before the rifle is even fired the first time.


So cleaning and lubing out of the way, Either the is now leaking between it and the top of the carrier, The gas block came loose and leaking between the barrel and block, or you got something in the carrier key/tube /block port to block one of the  passages instead.