Programs are frequently changed without warning. For example, BBC to America suddenly appeared one day on Channel 25. A month or two later, it was gone.
Also, there are times when you can't get NBC, or CBS or ABC. They the missing program appears again a few days later.
Additionally, programs are sometimes shifted from one channel to another, again without notice. Before you can be sure a program is really gone, you have to surf all the channels to see if it has been moved.