1. Check the mouse ball to make sure it is rolling, especially if the mouse is new.
2. Open the mouse up and remove the mouse ball. Inspect the metal rollers (there should be two of them, perpendicular to one another). Make sure that they are moving freely. Try moving them by hand and see if the motion registers by moving the pointer. This may fix the problem. If the inside of the mouse is dirty, clean it.
3. Try rebooting the system and see if the problem clears up. Occasionally glitches may cause the mouse not to work properly. If this happens a lot, you may want to update your mouse driver.