Since I cannot see what you are doing, all I can do is offer a suggestion. In teaching tying classes and the student complains about the thread getting shorter and tighter it is usually because they are not allowing thread to come off the spool as they are making their whip finishes. When you lift the whip finisher to remove the thread from the bottom notch, you have to allow thread to come off the spool to give yourself more slack.
I do not know if this is the problem, but, I had to throw it out there.
Warren
Fly fishing and fly tying are two things that I do, and when I am doing them, they are the only 2 things I think about. They clear my mind.