The cruise control speed can be reprogrammed for any desired speed at any time simply by holding down either of the set speed buttons.
Therefore, the most likely cause for this would be that the cruise control buttons have been held down while the trolley is stationary as doing so will set that button to operate at 0mph.
To sort this, simply use the forward button to get the trolley to the desired speed and then hold down either cruise control button until the trolley stops momentarily. After this, pressing said button will hold the trolley at that speed each time it is pressed.