Two possible causes:
First, the most likely: one or more of your TVs are optioned to the wrong tuner. Each set has separate tuners for use with an antenna or with basic cable (no box). You need to go into the setup menu and ensure you've selected the CABLE or CATV tuner, not the AIR or ANTENNA tuner. Then do a channel scan if the TV has that feature.
Another possibility: modern TVs can have an optional tuner (QAM tuner) for receiving unencrypted digital channels on basic cable. A TV with a QAM tuner could be receiving more channels than another one in the same home that doesn't have the QAM tuner. However, such channels are easy to identify, as they'll each have a suffix number, like 7.1, 13.3, 58.5, and so on.