Window sash front, left Beetle Cabrio 8/64-

The metal profile of the window sash clamps the window (with a rubber in between), this gives the fact the metal "sash" frame is under tension for whole of its life. Metal that's under tension wear out, as simple as that... call it "metal tiredness"... so after enough years of service the tension will fade from your window sash, mainly because of corrosion at the ends (that's where it starts mainly). Your window will not go down nicely anymore, will hook and turn... time to change the window sash