Why more lather some days than others?

I have really thick, coarse hair; I don’t know if that’s related. Some days, my shampoo really goes wild and lathers up beautifully. Sometimes, I have to use double the amount, because it just won’t lather at all. The same seems to apply to toothpaste. What kinds of factors affect this? Moisture/humidity? Temperature? It’s so weirdo. …does anyone else have this experience?