Washer not spinning through spin cycle

This problem has been happening for several weeks. At one point we were able to stop the washer, spin the knob from casual to white and the spin cycle would start just fine.

Now, when it gets to the spin cycle, it will spin for about 5 - 10 seconds and stop. But you can still hear the washer trying to empty water from the tub.

We are thinking it may be a balance sensor, but not sure if this model has one.

Any suggestions on what could be wrong?

thanks for the help.