DAG Active copy not with Witness?

I have a 2-node DAG, and I want the active copies to be running in Data Centre A.

If there is a failure event however, I want the active copies to move to Data Centre B.

If I just put the FSW at Data Centre B and set Data Centre A to have the active copies, then I see an occasional failover due to the wiles of the WAN and the databases will move to Data Centre B. In this scenario there will never be a failback to Data Centre A.

How can I say "run in Data Centre A if everything is working, otherwise move to Data Centre B"?

Thanks.