If your slides are not hydraulic, and are thus electric, it could be that the clutch brake is not set on the motor. Many times the motor brake is left off by the manufacturer as well as service facilities. I do not know why, but several of the service facilities I have dealt with were not even aware of the motor brake and I had to demonstartae to them what it is and how to set it.
The clutch Brake on the motor is found by locating the motor and then there should be a 3 to 4 inch long lever on the end of the motor, move this lever the oposite direction. You can check that it is engaged by pressing the slide out button, if the slide is fully extended and backs in a small amount when the button is released, the brake is not set, if it is set, it will hold it's position after the button is released.