I had one staff member using the simple view to record grades and that was putting in an override from the get go, so no one else could edit the grade. It wasn't recording dates against the grade either, so our marking block kept redisplaying the assessment for marking. Now that they know to do do that, no problems!
I had the same issue and Emma's idea of checking for grade overrides (in gradebook simple view) and unticking that override column, then going back and resaving the grades through the standard grading interface worked