Secondary D/G - Dropping Top Sash Window?

I have wooden sash windows in my flat, and uPVC secondary windows fitted inside, in the same two-part style. However, on a couple of the secondary windows, when the clip in the centre of the two parts is disengaged, the top half of the window plummets right down to its end stop, behind the lower part of the window.

I can see a mechanism in the side groove that looks like it should stop this happening (i.e. the window only moves as far as you move it, on a sash window the equivalent would be the sash rope and weight assembly) - it looks like a silver tube with orange rubber bungs at the bottom. On the ones that work a spiral rubber or metal cord comes out of the tube and links to the bottom of the window, but the ones that just fall seem to be missing this. How can I fix this, or (more likely) reattach the window to the spiral cord that has gone up into the silver tube?

Hope that's clear, apologies for the long post. Thanks in advance.