Exchange – no permission to logon

I have set up a few new mailboxes that have been frustrating me. I am using Exchange 5.5

Sometimes the account that I set as the Primary Windows NT Account has no permission to logon. Sometimes it will work in an hour or so. This time it has beendays. I keep changing it to the account I want and everything seems normal at the server side but still the user can’t open his mailbox.
Is there a way to force this to work?