There are usually safety switches associated with brakes, control arms, and PTO. The zero turn I use most, you have to take off the brake, then you can move the arms and engage PTO. I imagine you're familiar with any sequence you have to do. Maybe one of the switches is not working reliably. Do you have a volt meter? I would make sure the switches have good connections, and test the function. Give the mower model # and serial so people can look up that particular machine.