Couldn't this problem also be associated with the CYA level being too high? It would be interesting to know what CYA levels these problem pools have. If the CYA is very high, then it will take even more chlorine to reach the shock level.

Your typical pool uses the chlorine pucks in a chlorinator, and the chlorine pucks continue to add to the CYA levels, and eventually it gets too high. That's why most people on this forum avoid using the chlorine pucks when possible.