Sounds like it's possible that one of your RPM files has been corrupted in the download and won't install.
Hard to tell what is going on with out any error messages.
Try to run up2date from the command line. Often programs such as these will output extra information about what is happening onto what is called "standard out". which is basicly comments that get displayed to the command line. Unfortunately you don't normally see them if you just launch the app from a menu or taskbar in X.
another thing to try would be to use Yum
Yum performs the same function as up2date, and I prefer it. You should already have it installed, and it is compatable with running up2date on the same system. It's a command line based utility, but it actually fairly easy to use. You can also use to install new programs, and search aviable programs for keywords when looking for a program that you may not know about. Also 3rd party repositories such as Dag and FreshRPM's allow you to easily install programs thru their online repositories and that should still all work fine with up2date's little icon thing on the desktop. Shouldn't cause any issues.
To run these guys you have to become "root" or "superuser". Find the X terminal, should be in the application folder under system tools, although I prefer to have a launcher on my task bar which is a lot easier to get at.
Once you have that open and have access to the command line then you type in "su" or "su -" to become super user. This gives you administrative rights, but becaurefull there are now no restrictions to what you can do to your system once you become root. For instance you can delete the entire os by going "rm -rf /" and nothing will ever ask you if that's ok or not. It will just do it.
Then just type "up2date" to run the up2date command.
Then when you do that it should be able to give you something usefull, at least some errors. Or just use Yum and that may give you more usefull errors. Then it will make it easier to figure out what is going wrong. Like I said before I prefer Yum to up2date.