I press the "Install Updates", Update Manager goes on to refreshing the updates list but does not install them.
I can install the updates through
apt-get though. What could be the problem?
I assume you're talking about Ubuntu, and not Debian. The following answer assumes this is the case, although you might want to clarify your question a bit more.
First, it's a bug. Second, close the update manager, then in the upper-right corner, click on the icon representing the update manager, and select install updates. The update manager will appear; click on the install checkbox, and they should install.
If the update manager is not running in the upper right-portion of the screen, yet you have updates, it could be you have some file corruption issues that can easily be fixed. While logged into a Gnome session (regular graphical session), press Alt-F2, and type this:
Then in the terminal, type:
Close the terminal once it is completely finished. It may take a short while to force the issue.
I cut & pasted this command into the terminal and it ran and it updated several packages, but I don't have an Update Manager icon in my top panel, at the right or anywhere else on the panel (except in System > Admin > Update Manager, of course).
Should I in fact have the UM icon in the top panel at right? I'm running karmic on 64-bit ubuntu system75 Wild Dog desktop.