If you are referring to the AGITATOR, Whirlpool manufactured washers (such as Kenmore), use a split agitator design. The upper portion is called the auger, while the bottom is called the agitator. Between these components is a agitator cam assembly.
Since you mentioned that the agitator appeared to be working, this problem may be caused by either worn agitator dogs, a worn agitator cam assembly, or a loose agitator bolt. The following link explains what the dogs are and how to replace them:http://www.fixya.com/support/r3608577-replacing_agitator_dogs
The part number is included in the link provided. You can purchase replacements at pcappliancerepair.com for about $5. For a little more you can purchase the entire cam replacement kit, which includes the cam, the thrust bearing, and the dogs for about $11 or $12.
NOTE: You can refer to the excellent exploded view parts diagrams the web site has to offer for assistance. Whirlpool also recommends the application of a little grease on the thrust washer. The part number for their recommended grease is AP3098553.
If you have any questions about this repair, please post back with your complete model number (located on a nameplate along the wash tub opening under the lid) and let me know. I hope this helps you.
NOTE: DO NOT overtighten the agitator bolt. This can cause damage to the plastic, or restrict the agitator movement. Tighten until snug only.