It seems like the procedure you tried should have worked. You should check your Entourage database to see if there is corruption.
First, backup your Identity database:
In the Finder, click Go, and then click Home. Open the Documents Folder.
Open the Microsoft User Data Folder
Open Office 2008 Identities folder
Right-click the Main Identity (or your Identity name) folder, and then select Duplicate.
You have created a backup of your Identity and are ready to continue.
To start the Database Utility in Entourage, hold down the OPTION key on the keyboard, and then start Entourage. The Database Utility window opens.
Select your Identity then
Select the Verify database integrity option and click Continue. If problems are found, you can choose to rebuild the database.