One has to take his mouse, trackball, etc. apart maybe twice a year and clean it thoroughly. Everything on my trackball gets gunked up so bad the ball can hardly move. I've not been able to clean any pointing device satisfactorily without a full disassembly. If necessary, take notes so you'll be able to put it back together. Get some mild solvent (e.g. isopropyl alcohol) into the hub of the scroll wheel to free up the gunk and pick it out if necessary with a toothpick or straight pin. May want to put some rubber and plastic safe lube into the hub like dry silicone or teflon spray.
.bh.