That's what happened when I had my Maillard Helicomatic freewheel. I think it's just a problem with them- it won't shift just one gear- you need to shift two and back one. Helicomatic freewheels have little grooves at the top of the teeth, and these were supposed to act like the shaped teeth of modern cassettes and catch the chain. But it also required a chain with sideplates with sharp edges to shift properly, to catch in the grooves of the teeth. Those chains aren't made anymore. Plus it looks like the tops of the teeth are worn so the little grooves are nearly gone.

Dump the Maillard stuff. Get a new hub/wheel and put on a modern cassette/freewheel with ramps and cutouts. You'll be glad you did.