I bought the smaller version of this a while back for my butterfly photography and after much use (occasionally using more than the stated max rated weight
) the tension on the ball had become looser and the camera had recently started to drift down under its own weight once the trigger had been fully released, especially when used in an upright (portrait) position. Typically, any instructions which came in the box had hibernated in an unknown place and I was stumped as to whether it was possible to tighten it up again. After much searching on the web I found a thread which said that a single small allen key could be adjusted to fix this problem. The allen key is found at the ball end of the handle, just beside the trigger itself. I actually had to undo rather than tighten this allen key, but it has definitely had the desired effect and it now grips like new. That has saved me a few quid as I suspected I may have to ditch the head and buy a new one. 
Thought I'd pass this info on to others who may find it useful.
Trev