This is a typical problem with the Magic Mouse ... I used to do this ... get a piece of paper, printer paper if handy then turn the mouse over & press down so that the scroll ball is in contact with the paper & is given a good work out as you rub the mouse on the paper in circles & backward & forward is a scrubbing motion. The paper will dislodge enough of the gunk that gets caught up in there so that the scrolling works again ... but it's a constant I'm afraid. If you can upgrade to a Mighty Mouse I'd recommend this move.
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