First, check the easy-to-see parts.  Are the pins loose in the linkage rods?  Are the pins missing?  Are the linkage rods broken?  Do the linkage rods need adjustment?  All the linkage and linkage rods need to be reasonably tight and intact, because if the linkage train contains very much play, the control levers will hit the end of the hole in the control panel when you depress them before they get to actuate the hydraulic valve body and make the equipment work.  Control levers that don't all line up in the released position may need adjustment of the linkage rods.  If an individual control levers works fine one way but not the other (works up, for example, but not down), adjust the linkage rod.

You've fixed all the above easy-to-see parts, but the control levers still don't work properly?  Get underneath the control panel and check the axle shaft the control levers turn on.  If the axle shaft is bent, broken or held loosely, the control levers won't work properly.  When you depress the control lever, the axle shaft will be displaced sideways and the hydraulic valve body won't be actuated.  

Damaged axle shafts are easy to miss because they're usually covered up by the deck of the tow truck bed.  How do axle shafts become damaged?  Blown-out tires, wheel lifts that have broken loose from the truck, subframes that have broken in half, slings that rest on top of the axle shafts, and errant flatbed hose-track asssemblies all have the ability to damage axle shafts.  When you have to operate two levers at once to make one function work, suspect axle shaft problems.  Check for axle shaft disorders by manually trying to displace the shaft sideways.  A good shaft will not move sideways at all.