It is not recommended to work with the same project in the Family Tree Builder software on two computers.
The sync function syncs all the changes that were made in your project in Family Tree Builder to your online site and all the updates made in your tree on the site back to the software.
When you work with the Family Tree Builder software installed on two different computers, the updates you make in computer A will update the online site, but will not update the project open in the software on computer B (not even after a sync).
When you then try to sync your tree from computer B, it will not recognize the updates on the family site.
If you wish to work on your project with more than one computer, we strongly recommend that you install the Family Tree Builder software on your main computer, and use the online family site to edit your tree when working on your other computer(s) or mobile device.
This way, all the changes that you make using other computers, will sync back to your software on your main computer.