What new antenna did you try?
The manual says:
"Note: The ANT1400 may work well sitting on your TV or other components. However, because electronic components (like your TV
) can sometimes interfere with the antenna's reception, you might notice an effect on the antenna's
performance. Likewise, placing the antenna on a metal surface may interfere with reception. If the antenna isn't performing as well as expected, try moving it away from the component or placing it on another surface. Then, re-scan for channels on your TV or converter box."
I'm very skeptical about claims for an antenna being multi-directional. Notice that CH12 is almost at a right angle from your other channels.
If your tests tell you that the antenna needs to be further away from the TV, then you must believe your test results and go to plan B.
Other posters have told you that CH12 is on VHF, which is more prone to electrical interference, and your all-channel antenna may not perform as well on VHF as it does on UHF. I agree with them.