There definitely something wring with maybe the way the derailleur was installed as that Huret rear derailleur should easily be able to handle those big low gear cogs with no problem. Forum members with the same or similar Huret RDs on their bikes should be able to help as soon a they start chiming into this thread. Try changing the thread title to something that mentions "Huret rear derailleur" on it to snag those guys into this discussion.
Chombi