I had a radio that belonged to a friend, that the trigger spring broke. While I was in the process of fixing it, I noticed that the circuit board that sits behind the scroll wheel had come out on one side of where it mounts. This was causing the same problem you are having. Except he would have to push really hard for it to work.

Its not too hard. A handful of screws that keep the sides together. you need to be very easy when you pry up on the clear cover for the screen so it doesn't crack and you only need to remove it from one side ( its glued) a couple screws for the wheel and slowly split the cases. Set the circuit card back in the little slots and put it back together. If you are careful enough, you wont need to re-glue the screen cover.