i suggest trouble shooting starting with easiest to inspect or most likely to to cause the issue and work your way through the system.
assuming the buffer system is correct i.e. carbine buffer and spring for carbine length buffer tube or rifle buffer and spring for rifle length buffer tube.
1st is the rifle well lubricated? i don't care what kind of oil you use just oil the bolt and carrier liberally and then add more.
2nd is the gas key tight? does it wiggle?
3rd are the gas ring passing go/no go test? with the bolt carrier group completely assembled pull the bolt to the unlocked (extended position) and stand the assembly on the bolt face. if it collapses under it's own wight the rings are bad or there is a tolerance issue with the carrier.
4th is the gas block aligned properly? the easiest but NOT best way to test this is to plug one end of the barrel and put an air hose in the other end then place your finger over the gas tube and feel for air pressure. at anything over 30 psi you shouldn't be able to stop the air from pushing out.
after you have checked those things and ruled them out them you end up in the realm of slightly complicated.