More than likely the cause is that they lubricant on them is polluted with
dust, dirt, and other crap.
Use something like Radio Shacks "Component Cleaner". Its dissolves grease, oil,
dirt..etc. It also lubricates pretty darn well on its own. However, a little 3 in 1 after the
bearings are cleaned should be fine.
If after cleaning it still is bad... then the bearings may be bad, or the bearing races are
too nicked up.