I've had this problem before (I think I get what you mean anyway) and the way I've had luck with getting the knot to stay put is soak the little bugger in mod podge and glue it down on an old magazine, fruit snacks box, or something and leave it stuck there till its dry. Seems like if I have it glued to something securely (like seriously soak the knot and the end that isn't going to show in glue/mod podge) it seems to hold better. I hope you can get it worked out, things like this can be so frustrating.