the mechanism was a bit tired. The part of the inner frame that slides on metal ridges had groves, which I've filed out. I've also filed the ridges to be less sharp and bent them slightly to slide along a different part of the inner frame.

It's opns and closes pretty good now. Not perfect, but I wonder if it ever was perfect in the first place.

Eventually I'd like to replace the ridges with wider wedges, but I need to find a way to machine the first