Use one of the Wagner steam machines ( used for wall paper removal and general cleaning), there is an attachment that has a narrow point with bristles. The steam will soften all the crud build up in the track bunny fuzz. Once you have it sliding, remove the glass ( both the slider and the fixed portion ) and clean the track bunny fuzz thouroly. The steam cleaner will do this nicely, you will be amazed how much dirt has built up in the fuzz.
Sometimes just installing new bunny fuzz is easier. After cleaning, use silicone spray to lube the window track and fuzz.