Make sure your tracks are level, plumb and not too tight against the
door. Grab the stem of each roller and twist it, you should be able to
turn it freely by hand. Use your door manually to see it it binds at
any particular point along it's travel, then check the rollers again
while the door is bound to see if you can still twist the roller stem.
Once you figure out where it's binding you can make the approriate
adjustments. Then again you could also have a bad pulley or the cable
is not riding on the pulley correctly...

Rich
http://www.garage-door-hardware.com