On kenmore/whirlpool models they use a 2 piece agitator with a set of 4
Agitator dogs inside it. As the washer rotates back and forth, the dogs grab the inner walls of the upper agitator and that forces the clothes into a downward movement to the bottom of the tub. These dogs will wear out. To test whether or not these are bad just grab the agitator while in wash mode. The dogs should advance the top portion easily. If not then they are worn out. To replace them you must remove the agitator and sit it on the floor. Hold the base down with your feet and yank the top upwards. After seperating the 2 pieces you simply install the new dogs and line up the bottom with the top being careful not to break the little tabs on the bottom agitator that hold the top in place.