I've had this problem to. My solution was to use some DeoxIT D5 to clean the rotary encoder and now works as it should. I took the board out of the mouse but you might be able pack some paper towels around it to catch the over spray. Use the spray sparingly you don't need alot at all. One little splurt should be fine! Spray the top opening of the encoder then rotate it with the wheel to work it inside. Do this two or three times then clean up the excess. You can dab the top of the encoder to pull some of the D5 out to help it dry quicker. Wait 5 minutes before you test it. You could just put the parts together to test it out before you put the mouse together again, your choice. It should scroll up and down with out a problem now, your mileage may very. Good luck I hope this helps!