On some computers I have a problem. The down arrow at the right of the quick access toolbar doesn't show. I checked that my interface is not lock too. So to customize the quick access toolbar I have to use the CUI.
I don't know if it's related, but my customize toolbar (from my partial menu) have to be displayed manually each time I start AutoCAD but not my custom tab in the ribbon.
Would it be possible for you to attach a screenshot of the behavior you are experiencing? This would all me to further troubleshoot this issue.
The problem is resolved. It seem related to graphic screen resolution and a workspace not saved correctly.