Software Update can't find updates, but Application Installer and yum can

Recently, Software Update has been unable to find new updates for my machine. It will open up, the progress bar moves, and then I get the message that there are no updates available for my system. However, checking for updates through Application Installer or on the command line with yum works properly. If I use one of those applications to check for updates first, then Software Update is able to install new updates.

Another weird thing is that when this started, Software Update now opens in a maximized window instead of its usual size.

I am running RHEL 7.6.