You may be encountering multipath, Paul. I'm in multipath central, mid-Manhattan, and rarely use my book-size Silver Sensor antenna because of it. My set, a Philips RPTV, has a special mode that creates the equivalent of an on-screen spectrum analyzer. All the multipath reflections from stations appear, dynamically, as green peaks along the X-axis, changing in height as I move the Silver Sensor only inches. Several other readouts appear at the same time, indicating signal presence or 'lock'. Some antenna positions give an apparent strong signal, but I can also see horrendous multipath reflections. When I switch out of tuning mode (unfortunately, only by turning the set off), I can't tune in a strong-signal strong-multipath station, which might apply to you, too. Some of the latest-generation tuners, such as the new Zenith, might solve tough multipath problems. Moving an antenna to a roof could make a huge difference in signal characteristics.
I'd start by stripping away all the hardware not needed. Haven't adjusted a big roof antenna, but sure hope you've got two-way communications so someone can instantly view the results of antenna movements. Slight changes in antenna elevation, others have reported here, can make a difference. You may well need a rotor, or separate antennas for different stations and a common lead-in. If it is multipath, many have pointed out here that relocating antennas only inches can make a huge difference. With my interior setup, I have separate antenna positions outlined atop my RPTV for numerous stations. But most, after the 9/11 attack and the World Trade Center destruction, no longer apply. -- John