Why isn't the Excel Update function actually updating the record in Sugar?

I would like to use the Excel Update function to update a field on multiple accounts. I extracted a list of account records, including the ID, using the Query Wizard.

The field I want to update is the Assigned_User_Name, so I typed in the username of the person I want to re-assign the record to. I then highlighted the row, and selected Update. 

It said "Performing Update", then "Update Complete". But when I look at the record in Sugar, the assigned to user has not updated.

What am I doing wrong? Is there a way to troubleshoot what is going wrong? I have checked the obvious settings (I am logged in, the instance URL is correct etc).

I am using Ultimate edition, v7.2.1 hosted on site, with Excel 2013.