A repairman told me some years ago, when my dishwasher failed to "move
along" and finish the program, or took an eternity to finish, that an
inadequate amount of water caused by scale was the problem. If the machine
isn't getting the amount of water is requires, it won't get on with it.
Don't know if this is true; it's just what he said. After he de-scaled, it
worked like a charm.