The problem is the locking off to length of the top arm. The arm locks to the chosen length with a triangular black knob. Mine rotate (very jerky), but don't stop the arm sliding. Inspection inside the arm with a torch, still doesn't let me see how the flat plate inside the arms channel and attached to the black knob is supposed to work.
My next plan is to have a good look, by taking the arm completely off when I get back home, however this seems a major task.
I've gone a long way back through the forums, but as yet can't find any similar posts.
Has anyone else had my problem? Any tips?
Cheers Ron