You have received some good advise and techniques to try. I have not tried this idea, but, I would suggest this:
Before making your dubbing loop trap the tip of the aftershaft on the hook shank with a couple wraps of tying thread and then make your dubbing loop. This way the aftershaft feather should stay in place and all you would need to do is slide it over into the dubbing loop and then twist it up. Like I said, I am not sure this would eliminate your problem because I have not tried it, but, in my mind, it sounds like it would help and should work.
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.