Seems lije you have your trakball a little bit drty on the trackball rollers. I had this probelm as well but the other way around, I could move it vertically but not horizontally.
Get a small phillips screwdriver to open the metal rim around the trackball. Do it gently so the trackball sistem doesnīt go flying right out of its place. Then get the trackball and the rollers out, clean the rollers and then assemble again.
I know there is a link with picture explanations, but canīt find it right now.