I start my week with 7 or 8 pipes, cleaned and ready to work. There is always 1 pipe that for some reason just doesn't make the grade so at the end of the week, it gets rotated out of the group and a different pipe get subbed in. Then a month or so later, it finds it way back into the group and smokes fine. Why? What changes? Sometimes it takes months before a pipe falls off so to speak.