Well, I finally succeeded today. The first time, I didn't have a camera set up. The second time, I had the camera set up but discovered in the middle of the recording that my camera doesn't zoom out once you've started recording, which led to a lot of confusion and motion in that recording. So this video is of my third success today.
I think the key for me was finding a good mullein stalk. I did almost everything the same today as I have done in the past, but with mullein.
Here are a few other suggestions that seem to have helped me. The spindle should fit easily into the hole, not too tight. The notch should extend to about the center of the hole and be wide enough to allow the powder to fall out easily. About 1/8 of a pie piece seems to be a good rule. An important part of the process seems to be using the float technique. Use it at the start for two reasons: first to build up a pile of powder in the notch and heat in the hole without exhausting yourself, and second because applying less pressure allows the powder to be finer which helps it ignite at a lower temperature. One change to my technique that I tried today which probably helped me succeed is that when I began to get exhausted, I dropped back to a slower spin using floating so that I could keep the hole warm while I regained some strength. Once the powder has filled the notch, put all your effort into spinning fast and hard several times. Then, when you see that the smoke is pretty strong, stop spinning the hand drill, but leave it in the hole. I think that leaving the hand drill in the hole may contribute by keeping the heat down near the powder, it may help the powder ignite if it hasn't already. Then after you've stopped spinning for several seconds, if you see that the smoke stops, you can start spinning again without losing much of the heat. But if you see that the smoke continues, take the spindle out carefully so you don't damage the coal.