Antique Elgin 17 Jewels 10krgp Ladies 313 Watch Manual Wind Runs But Problem

This is a fine antique ladies' watch, it runs but something is wrong with the winding stem. It just keeps turning but doesn't turn the mainspring. However, when the mainspring is manually wound the watch runs fine. I'm not sure what is going on but I don't have the knowledge to fix this. So my loss is your gain. You can get this watch with 10k rolled gold plating and expansion band for a bargain. Hopefully you can fix it.