Here's my two cents, I'm no expert but have made a fire with a bow in drill probably 50 times over teh years.
For materials, I highly recommend dried yucca stalks. Try to find some larger-based pieces for fire boards, and straight spindle pieces in the six inch ballpark. I have tried but never been able to start a fire with willow and cedar. I recently read on here western red cedar is great, found some at a lumber yard and bought some I want to try out sometime. It has the right "feel" like yucca, lightweight, feels pithy but crumbles instead of compresses (like willow did for me). At my work we have some grandpa yucca plant that had a seven foot stalk on it...I couldn't resist, I snuck in a knife and collected that one mid-winter.
The tighter to bow string the better it grabs without slipping. I usually have it tight enough that it is difficult to get the spindle in the bow and difficult to keep it from springing out, but once you get in your groove it stays in place relatively easy and the extra tension helps with slipping. Hopefully more tension will help your slipping problems. I have made my spingles non-round, think the texture of a rough-whittled stick or an octagon, the thought being the texture on the spindle may help it grab and not slip. I wouldn't swear by it; smooth works.
The fact that you have lots of smokes means you are on to something and really close. It is a fine art to make a notch in your base board that will let out enough dust to make an ember. Are you not getting much dust out? Probably need a larger hole, or need to keep going. I can't imagine the ember going out if you add a few more seconds of hot dust, if you have the strength left. Lots of brown/black dust but no ember? Probably need a smaller nock, or one that doesn't go as deep towards the center of your drill hole. Mine usually goes in halfway or slightly more from the outside of the drilled hold towards the center point/bottom of the hole, or roughly 1/3 of the way through the hole.
Don't give up, you'll find your nack. One morning while camping with a dear friend and experienced bow and drill user we decided to make our breakfast fire with the bow and drill. I don't know how many times we tried and failed, it was at least 10 tries, but eventually gave up and used a match. A similar kit used later was reliable enough I used it for demonstrations with great success.