how can i retain bbc 1 and 2?

my thomsondtr model no dti6021 will scan ok but it will not retain any bbc channels. for instance when i switch to sky news and then switch back to bbc 1 or 2 the show blank whereas all other channels work fine. i have scanned and re scanned and the same thing always happens. just tell me how to retain bbc channels

don bell
March 2010