When you browse to a user and make an assignment in the Encryption Users section of ePO, the process appears to complete. But, the user object doesn't appear in the list of objects that have been assigned. There are no errors when you add the user.
NOTE: Users are added in the ePO console by navigating to Menu, Encryption Users, Actions, Drive Encryption, Add User, Select Use, and then clicking OK.
Eventually, the user might get added, but it can take several hours to more than a day.
After you restart the ePO Application Server service, adding users might work successfully, but only for a short period.
There's a resolved issue discussed in
KB91776 - Unable to add a user to the list of Encryption Users or user appears after a delay. This article or the present one might apply to the issue.