the problem with your setup is: even though Media Folder in all of your accounts point to /Users/Shared/iTunes/, the iTunes Library database is still located in ~/Music/iTunes/ for each account.
What you need to do is:
1. as admin, reset Media Folder location to default location ~/Music/iTunes/iTunes Media/, then consolidate all the music in that folder, so that you can delete /Users/Shared/iTunes
2. quit iTunes, and move (not copy) ~/Music/iTunes folder to /Users/Shared/
3. in Get Info window for /Users/Shared/iTunes set permissions for everyone to Read & Write, then Apply those permissions to enclosed items (this option is under "gear" icon)
4. create an alias to /Users/Shared/iTunes in ~/Music/, the alias should be named exactly iTunes
5. log in to your other accounts, delete the folder ~/Music/iTunes and perform step #4