When signals are blocked by terrain, the results become less predictable. That you see one station but not the other is also understandable when the antenna is pointed so that it's perpendicular to the ideal aim. Experimenting with mounting location, height and aim may locate an acceptable position for the ANT-751. The San Diego market can be a single antenna market for some viewers, but many need two antennas.
The ANT-751 sounds as if it's serving well as a UHF antenna so consider adding a High-VHF antenna such as an Antennacraft Y10713 or Winegard YA1713. You can combine the UHF and VHF signals with a UVSJ. Mount the UHF antenna at least 3 feet above the VHF antenna, greater separation is ideal.
http://www.antennacraft.net/Antennas/AntennasVHF.html
http://www.winegarddirect.com/viewit...13%29&p=YA1713
http://www.amazon.com/Antennas-Direc...f+vhf+combiner