Ok. You guys have convinced me to go on a mission to find a bad chalk. I found an old cube of Silver Cup last night. I'll put it through the tests this week.About 10 years ago I had a student who had his own table. Being a 49ers fan, he had covered the table with red cloth and had red chalk to match. I avoided vomiting and began the fairly basic lessons. When we got to draw, he kept miscuing. I said, "Let me show you how the stroke should look," and proceeded to miscue several times. The chalk he had simply did not work well. I got some other red chalk for the next lesson as did he. None of the red chalk worked for either of us, although blue chalk (which leaves really ugly marks on red cloth) did work OK. The student changed his cloth to green and his chalk to blue.
My conclusion: some chalk does not work nearly as well as other chalk.
Maybe bad chalk is hard to find but I suspect it still exists. I've encountered some really bad chalk in taverns.