If your washer doesn't agitate, check these:
If the lid switch is defective, the washing machine may not agitate or function at all. The switch is inside the washing machine main housing near the door frame. Often you have to raise or open the top or front of the washing machine to get to the switch. If it's defective, you need to replace it.
Many washers produced by Whirlpool® use a small, relatively inexpensive motor coupling. It's plastic and rubber and is mounted to the shaft of the motor on one side, and to the transmission on the other. Over time, the coupler wears out and fails. If this happens, you need to completely replace it.
Many washing machines have one or two belts. If a belt is broken or badly worn, you need to replace it with a genuine belt from the manufacturer. (Some washing machine belts are designed with special characteristics not found in automotive belts.)
If your washer was made by GE®, it may use a clutch for agitating the clothes. As the clutch wears out, it may prevent the washer from agitating well or at all. If the clutch is worn, you need to replace it. For this job, you probably want to hire a qualified appliance repair technician.
Many washer brands use a reversing motor. For agitation the motor runs in one direction, for spinning and draining, the other. It's possible for a motor to burn out in one direction and continue to operate in the other. If this happens, you need to replace the entire motor.
The motor or transmission drive pulley may be worn and unable to turn the drive belt. If so, replace the pulley.
The transmission could have either of these problems:
Older washers produced by Whirlpool® have a transmission with an electro-mechanical shifter. If the shifter becomes even partially defective, the unit may not agitate properly or at all.
The transmission may have a worn or broken gear, or some other internal problem.
If you suspect a transmission problem, you may have to call a qualified appliance repair technician to repair it.
The inside of the agitator--where the transmission shaft attaches--can become worn, and strip out the spline that allows the agitator to properly grip the shaft. Then the transmission shaft rotates back and forth as it should, but the agitator doesn't move properly. If this happens, you may need to replace the agitator and/or the transmission spline.