Can't side scroll on new MacBook trackpad

I am unable to use a two finger scroll to scroll sideways on my (new model) MacBook Pro. I can do vertical scrolling in a window, but not side to side. I used to be able to, but one day I just noticed I can't.

I've checked in Safari, Finder, Preview, and iPhoto. These are times there is a left-right scroll bar present. The option is checked in the Preference pane.

So basically, if I'm looking at a photo, I can do a two finger scroll up and down, but I can't go left and right.

Any ideas?