Scrolling down w/ trackpad not working

I have "Use two fingers to scroll" enabled in System Preferences and until yesterday it worked just fine. (Note: I don't have a Multi-Touch trackpad.) Now, for some reason, I can't use two fingers on my trackpad to scroll down. Scrolling up with two fingers on the trackpad works correctly, as do the up/down buttons on the keyboard. It's just scrolling down with two fingers that isn't working.

Does anyone know what causes this or how to fix it?

Thanks!