Lift your washing machine lid after the spin cycle engages. Is the washer basket spinning? If it isn't, the washer-basket nut could be loose, keeping the transmission from being able to turn the basket. However, the spin cycle can be interrupted for a number of reasons. If the water drains during the spin cycle, but the washer does not spin, the washer-basket nut or the transmission beneath it could be the cause of the problem. If the water does not drain, and the basket does not spin, the drain, pump or other mechanisms are possible sources of the problem.
Lift the washing machine lid midway between the wash cycle and the spin cycle. Does the washer basket look like it just stopped agitating? Did you hear the washer basket moving rapidly before you lifted the lid? If the basket does not agitate, the washer-basket nut could be loose, keeping the transmission from moving it back and forth. But there could be a problem with the transmission itself. Other possible sources of the problem could be your washing machine's lid switch, the motor or the motor belt.
Stay in the room with your washing machine throughout all of the cycles. Listen closely for a repeated clicking noise. This clicking noise may be a sign that the washer-basket nut is loose. The clicking is the sound of the transmission trying unsuccessfully to turn the washer basket.
Most people lack the expertise to diagnose a problem with the small components of a washing machine, such as a washer-basket nut. A washing machine can develop a problem for a number of reasons. It is best to call a professional to accurately diagnose the problem. If you have no doubt that the source of your problem is the washer-basket nut, do not attempt to replace or tighten the nut yourself. The interior of your washing machine is full of sharp metal edges. While attempting repairs you may hurt yourself. At the very least you will likely void the washing machine warranty.