From your description, it sounds like you are doing everything exactly right. I can't think of what advice to give you other than, "Keep Trying." It may be helpful to shift your WBTB a couple minutes in either direction. Play with the timing a bit to try and nail a REM cycle.

You could also try taking a supplement / drugs to help your brain chemistry. It sounds like you are fine with self-medicating Try a Galantamine/ Choline combination, as it seems to be the most effective of anything out there. I can pretty much guarantee that you will become lucid if you combine your current routine with Galantamine.