Sometimes, when I do Alzapua, I can not produce clean sound, there're still I heard my thumb's nail picking. Usually, I use it while I'm playing (or learning with) some regular chord, like E major. Well, I do have long thumb nail because I want to learn that flamenco technique, because I think it'll be easier doing it while you have long nail.
Am I doing wrong step? If so, is there any method that I can try with?