Probably putting too much elbow grease like bob 48 says - OR - Your heart board is too soft or drill too hard is my guess. I'm not familiar with horseweed, but I usually stay away from any woods that have pitch if your using pine. That's just my practices, not saying that those wood wouldn't work. You should be getting a few coals out of each "hearth" though. Humidity and elevation are big factors of course to consider.
Also, in my 6 years of using the drill method, if I don't see jet black dust piling up, I stop because I will NEVER get a coal. You can get smoke all day and never get a coal. For me it meant I was using material that was not at the proper moisture content.
I use elderberry drill with a buckeye hearth. Both materials for each tool works too. I find that just as each wood has dried out completely, that this is the best time to use these woods for fire making. Too dried out and the wood does not like to make a coal for some reason. I think the morphology of the wood changes and inhibits flow of oxygen??? Just a guess.