I just solved the same problem and believe me the solution was REALLY hard to find.
Go to Grades>Categories and Items (If you only have a drop down list, go to Grades> General Settings in the your site Administration and select Tabs and Drop Down List)
Go the Items on the list and select Edit on the first item. Don't try doing a select all - the solution only works if you apply the changes to each item individually.
Select each item and make sure that you have set a pass mark and unticked the Locked check box between the date fields.
Try it and see if it fixes your problem.