Activity priority - by: Lee

Hi,
I have project, subproject.
Only Subproject have activity.
I try to prioritize activity from all of the project and other project, but it does not work.
All activity have same characteristics, and a priority to 500.
I put 400 on this activity, no change on the diary.
Il put valid and request date to the date of the work.
I can't change WBS project and activity…
Do I miss something ?
Thanks.