the black knurled **** would be the manual lowering valve and generally you would turn it counter clockwise to open the valve or maybe pull on it.
What i would be checking for is the electric solenoid valve in the lowering manifold block which is usually located in the same place the manual valve is.
If you have checked the lowering switch circuit and it is turning on and off correctly and you verify this is not an electrical problem then go to the electric solenoid valve on that manifold block and remove it and check it to see if it is stuck open or has debis in it. If there is not a separate manifold block like what i speak of then it may be located on the pump assy. Being an orderpicker i'd bet it has a separate block for lowering control.

Some of these cartridges have a small screen on the end covering the openings and sometimes that will come apart and the metal debris from that can cause the valve to hang open.

Note: when removing the solenoid coil and cartridge valve make sure the platform is fully lowered and be prepared to catch some oil as it may start to gravity feed oil from the tank and that can wind up being a huge mess if you do not have anything under it to catch the oil. One way to insure it does not gravity feed from the tank is to pinch off the return line with some vise grips, just be careful not to damage the rubber hose by squeezing it too much.
And most importantly if you are unsure of any of this please call a tech in to do it. ;o)