In FUB, you must exclude the following from your list to match up with Brain:
Exclude anyone in stage of Trash
Exclude anyone with no email
Exclude anyone with tags of Unsubscribe or Unsubscribed
You should match up REALLY close, usually within 10-15* people and this is a clean sync.
*this can change due to the number of the list
If you have followed these steps but are still having issues, feel free to contact our Support Team via the chat tool in the bottom right hand corner of your platform.
Related Article: Follow Up Boss Support