See this Comm Center thread in this area where Dick Berg shows Peter Brackenridge why his flats aren't working. Add info on this to ACP Help. lso make sure it is clearly explained, including the included test data, in the Web UI slide-out help.
The test data etc. was covered in the slide-out help, but I moved that info right onto the tiddler itself. People just don't like to click help and in this case there is a lot there to know.