iMac OS 10.6 won't restart or shut down

When I try to shut down or restart, I get a message that says other users are logged in and may lose data etc. OK that's normal, since my wife also has an account on the computer and is usually logged in when I am. However, when I enter the username and password (either mine or my wife's), nothing happens when I hit the shut down or restart button. If I am the only one logged in, the computer shuts down and restarts OK. I used Onyx as well as Disk Utility to run permissions repair and other maintenance tasks such as cleaning caches, and Onyx says the disk is OK. The only thing that works to shut down is to use the start button. What else could be happening?

That's normal. Other users who are logged in should log out first before you (as the admin) can effect a normal shutdown. Why is your wife logged in if she isn't using the machine?

Forcing a shut down by the use of the power button can eventually lead to data loss.

Enable Fast User Switching, then you can quick change users and log here off.

thanks but still ?

Thanks for the tips. I'm still puzzled as to why I used to be able to shutdown/restart by entering a user name and password when we were both logged in and now can't. Something changed--maybe when I upgraded from 10.5 to 10.6?