I had a problem where I could receive a station when a small antenna was sitting on top of a bush at 5 feet, but a larger antenna at 20 feet could not see to station at all. It turned out to be interference from an FM station. Apparently at the lower height the FM signal was blocked by terrain or trees, etc. I was able to see the signal using a cheap $22 SDR dongle. The fix was a DIY High Pass Filter to suppress the FM band. I detailed everything at
WT4Y.com.
Sorry for the delay to get back to this. Crazy day, and multiple vehicle problems as well...
My parents had a similar problem back in the analog TV era. They had two closer, strong FM stations in the same direction as a distant VHF TV Channel 6. My friend mentioned above made custom 2-notch FM filters for several houses in our neighborhood.