Distance joint - change distance from script

Hey guys,

I am trying to create simple crane, on the end of crane is box connected with distance joint (pic. A). Distance of this joint is managed by script. What is strange, that sometimes box is moved and connected correctly (pic. B), but sometimes happens, that only gizmo (in scene view) of distance is changed and box is still on old position (pic. C)
Is there something more, what is needed for distance change?

 

Looks, it is happening when box is grounded. In case, that box is starting in the air everything works fine