If the bottom agitates and not the top, the problem is likely the 4 "agitator dogs" designed to allow the top agitator to turn only one way.
On the Sears parts site:
http://www.searspartsdirect.com/partsdirect/getModel!retrieve.pd?pop=true&modelNumber=110.26902691&pop=flush
replace the 110.26902691 with your model number (lift the lid and the model number should be printed on the cabinet) and look at the section on the Sears site for Agitator, basket and tub.
This is what they look like:
#21 on diagram
Dog, agitator (4)
Number:3366877Substitution:80040
$9.25Remove the fabric softener unit, lift the top agitator cap (use a screwdriver) and you should see a plastic part in the center of the top agitator with a 1/2" square drive.
Remove the drive and lift off the top agitator. Turning it over you should see the 4 "dogs", replace these and the top agitator should turn in one direction only.
Replace the 1/2" drive, the cap and fabric softener unit and the top agitator should work.