I once had this problem with my analog cable television. The cable guy (cable company omitted because it doesn't matter) came and tested our reception; I wasn't getting the same signal strength on all channels. Turns out a squirrel had chewed through the cable between the pole and my house.

Another cause may be a scrambled channel. Many scrambled channels on analog cable no longer use the easily defeated VideoCipher system and use instead their own proprietary system.