Most common problem, is grease that has been poured into the tracks, to which all the dust and sand then sticks to, especially the lower one!
Try cleaning out all the muck & grease from all the tracks and then just ensure that only the bearings for the rollers are oiled, many people think it needs grease/oil in the tracks themselves - just like you would grease the road you drive on NOT!
This page on the mech diagrams might help takes a little studying and using the table and descriptions in the top left to identify parts - clicking the numbers on the diagram will open the table as well.
http://www.lushprojects.com/bongopartsm ... o=.html#20