For some odd reason, many of my flies are becoming undone right where I use the whip finish. I have a whip finisher tool, and I sometimes do it by hand as well. I just dont think that it is right for the fly to become unravled after I whip finished it down. My best thought is that my heads are not very good, so therefore, the whip knot slides down and onto the hook eye. What can I do in order to get a better whip finish knot and to keep it hold?