In the case that two users have a certain role assigned to them, but one does not have access to (example:) all of the companies in the system, permissions may have not been appropriately set even though the user is given a role.

Go to Users in the Administration tab.

Highlight the user and click on Edit.

Permissions can be found below the User Details (red box in the screenshot below). In this case, you would be required to tick all the companies listed for the administrator to be able to have access to all companies.