Two steps below taken from first instruction booklet for 42114 set. Since one end of clutch is locked it basically tells, that under normal conditions, clutch should not be spinning at all. Clutch is used as a safety feature within the build, when something get stuck, not to cause further damage to gears or human flesh. So in this case it should only begin spinning if the rest of gear train is having issues.
I'm not entirely sure what could be causing your issue of bed not operating, but I suspect one other tricky place could be the reason. Please pay attention to location of 20 tooth bevel gear for each side (highlighted in blue) in pictures below. One of these needs to be on top on one side and on the bottom on the other. If placed in same direction, one side will pull while the other will be pushing.