take off you side covers, and look on the back of the pulley wheel as you turn the engage, disengage switch. once you see how the mechanics work it will be easy to see where the problem lies. that switch is often bad. I have had some bust on me.
if it will not work anymore you can permenantly put in engaged mode by drilling a hole through the pulley wheel, and pinning the arm in the engaged mode with a nut and bolt.