Like Larry said, you just got to work on the tension used to tie the Sebile Knot. It does work, and very well. I don't use the knot much because I created my own know based on the Bimini loop, and while I think I created it, because it is a modification of an existing knot, I sure someone else would take credit for it.
One thing I know is that the Sebile knot takes three hands to tie and mine takes 3.5. LOL
Spend some timing experimenting with the Sebile knot at home until you can tie it in your sleep. Additionally, look at alternate methods to finish the knot. I don't like the overhand knot option, but the one I use I learned while finishing off my archery strings. I have no way to explain it, but there are other ways to finish the Sebile knot.