Have you tried compressed air? I even went so far as to use my compressor from the shed and blow out the scroll wheel area, holding it upside down, whilst "scrolling".
Sometimes it's just a bit of dirt.

"If it turns out that President Barack Obama can make a deal with the most intransigent, hard-line, unreasonable, totalitarian mullahs in the world but not with Republicans? Maybe he’s not the problem."