Terrain (i.e. hills and ridges) isn’t the only thing that will block line-of-sight radio signals such as ADS-B. Buildings and houses will too. So will trees to a lesser extent (they’re not solid objects like buildings, there are small gaps between the limbs, branches and foliage that a few ADS-B messages may get through.) If you live in a one-storey bungalow with your antenna on the roof and most of your neighbors live in two-storey McMansions, there’s your problem.
As you mentioned you’re using a FlightAware Pro SDR dongle (which is the same kind I have, the orange one with a built-in LNA but no filter) also see Do I Need A Filter? I don’t think interference would affect maximum actual range as much as it would overall message count and aircraft seen, but I suppose it could do. It doesn’t hurt to check. Being in a highly-populated area like that I expect there are cell phone towers everywhere so the odds are that you need a filter anyway.
How high is your antenna, how long is your coaxial cable, and what kind of coax is it?