Why can I receive UHF HDTV channels but not VHF ones?

[PatrickHenry599]

My zip code is 78249, San Antonio, TX. My tv is HDTV ready. I installed an indoor Phillips amplified antenna model SDV6122/27. The box says for HDTV/UHF/VHF. AntennaWeb show all the HDTV channels I want to view as being in a violet zone. I receive crisp and clear pictures on my channels of interest that AntennaWeb shows as violet/uhf. The two channels I can't receive are shown as violet/vhf. These are KSAT-DT 12.1 AND KLRN-DT 9.1. All channels I want to receive are located at the same approximate distance and compass bearing.

Any information why I am not able to receive the violet/vhf channels and suggestions on how to do so will be appreciate.
Thanks!

[tigerbangs]

why-predicting-reception-with-indoor-antennas-impossible-t1024.html