I too have a scroll wheel that will not scroll down, but does go up, left and right. Intermittent at first, it has gotten worse over the past few weeks. The "feel" of the wheel is different when attempting to scroll down, it does not offer resistance as it does when scrolling up, left or right and does not make the same loud clicking sound. I tried re setting preferences, unplugging and re plugging, re starting and even calling Apple. Apple support told me my serial number could not be right because the serial number should be 11 characters but mine was 13 etc etc. This took 15 minutes of going back and forth. Finally they suggested that I buy phone support for $45.00 or take it to a store - but I bought it direct from Apple (draw your own conclusion about how helpful this was). I tried compressed air spraying it directly into the area around the scroll wheel with no result. Finally, I got a wash cloth and pressed down REALLY hard on the little wheel and scrolled - trying to clean the thing. After about 20-30 seconds of doing this, it started working again.