Our wood stove has a 8" opening & we have a 6" flue.?

We used an adaptor to connect the two together near the top of the stove. We are experiencing backdraft fairly often. We were told that extending the length of the chimney on the roof would not help. We were also told that changing the adaptor (8 to 6) from the top of the stove to closer to the ceiling would not help. Are there any fixes other than buying a new wood stove. This one really heats our house nicely. Thank you for any help.