Creating new mailbox stores should be straight forward, right?
Strange one encountered in my lab yesterday, created about 7 stores and the 8th one created but the Master was pointing to the server itself and not to the DAG.
Try mounting a store in this state and you get the following error:
Initially I thought that replication has not finished but gave it more time while I completed the other store copies and circular logging.
I ended up deleting the store, waiting for 15 minutes and adding it back again, this time it added fine and the Master was pointing to the DAG and not the server.
Hope it helps.