Make sure the mounting is solid and perpendicular. Most of the stations in the LA area are on Mount Wilson just north of the Pasadena-Altadena area. Antennas should be pointed in that general direction. Tree growth or obstacles in the line of sight between you and the transmitter site on Mount Wilson. Check with some neighbors to see if they are having the same experience with reception.
Older sets can experience tuner problems that can cause single channel problems.
With new TV's this probably not the case. If you have a recent model set you can rule out the tv. This channel is usually reserved for simulcast in Spanish or for Descriptive Video narration for the visually impaired. There should be a button to turn off the SAP channel either on the front of your TV set or on the remote control. DVS provides narrated descriptions of a television program's key visual elements without interfering with the program audio or dialogue.