Dan,  The Stella winding and locking mechanism, as with the entire
Stella movement, was "over engineered" in my opinion, although it does
work just fine when you understand what is going on.  On the 17-inch
Stella there is a small planetary gear that meshes with and revolves
inside the large internal winding gear.  When tension from the springs
is sufficient the small planetary gear will lock into place via a fixed
horizontal post on the casting.  I suspect that your planetary gear has
broken or is missing and will need to be made.  Hope this helps.

Doug Olds