Ok from the picture the bolt head and lock looks square and not rounded. If either the stop or the head are rounded you might try filing or grinding them to bringing them square.(yes this is yell on your paint).
Another thing are you sure the rope is on the winch spool the correct way. So the locking mechanism will hold.
I have never seen on of these that will not hold.
Thanks. Not worried about paint, just a rattle can quick splash to hold down rust and get to the show next weekend. I configured a temp fix for now by just inserting a shaft through the spool hole with a clinch pin. The rope is would correctly, I think the configuration is just worn out and needs a refresher