I've got (what I think is) a Huret Eco rear derailleur which seems to somehow get "stuck" in an "open" position when shifting on the front chainrings, i.e. the return spring somehow stops "working" and the huge amount of slack in the chain resulting from this causes the chain to fly off the chainring; How can I ensure that my setup is "correct" for this model? When in its "stuck" position, it looks like in the following pictures:
In the pictures above, the retaining spring is highlighted in red. While the derailleur is stuck in its "open" position, if I gently push the derailleur arm forward with my hand, it then "snaps" closed, keeping the chain properly taut:
What could be causing the derailleur to get stuck in this way, and how do I fix it?