Can't seem to end date benefit group with data loader

Summary:

I am trying to use the PersonBenefitGroupExcelLoader to end date a large number of Employee benefits groups but it is not making the change in the system. Can anyone tells me if I'm doing something wrong or if other people are having this problem

Content (required):

I have taken the following steps

1. Opened the spreadsheet template from the system
2. Clicked End Date in the toolbar
3. Added the desired end date in to column I
4. Double Clicked Mark for Deletion
5. Clicked upload in the tool bar.

The status then confirms that the action was successful but when I check the system, the end date does not show