What kind of antenna are you using? Multipath, which caused ghosts in the analog era, could prevent the station from being decoded properly even with a good signal. A highly directive antenna is a must. Easy on UHF, even on indoor antennas, tougher on VHF if you're not allowed an old-fashioned Channel Master outside antenna and rotator where you live. Rabbit ears, even those with a preamp, probably won't work.
We have the same issues in Phoenix, between the mountains, airplanes in some areas, and tall buildings really messing things up even in strong-signal areas. Too bad most TVs don't display signal quality as well as strength when you press the Info button. I have one 2009-vintage DTV-to-NTSC converter that does, but none of my TVs do.
Another suggestion is to install an FM trap between the antenna and preamp (if one exists). If you're in an area with 100 kW FM stations, they could overload the preamp and cause all kinds of havoc.