First, unplug your tablet, restart Photoshop, and plug the tablet back in. Wait for a few seconds and then try changing your settings.

If that doesn't work, I've had some brush dynamics problems in the past that were caused by the Windows Tablet PC service. If you're in Windows, try turning off the service: Windows-r will open the Run dialogue. Run services.msc. Find the entry "TabletPC Input Service," right-click on it and choose Stop.

The only other thing I can think of that might be a problem would be a corruption in your preferences file. I don't recall the procedure for Windows, but in MacOS, you can recreate the preferences by holding down all of the modifier keys (Cmd, shift, alt, ctrl) while starting Photoshop. It actually only requires three of those keys, but I can never remember which ones, so I just use all of them, and it works. However, resetting the prefs will unload custom brushes, restore the original workspace layout, and undo any other customizations to your installation. It won't uninstall plug-ins, though, since those aren't usually referenced in the prefs file.