I noticed this the other day, i am using a Tivo HD with cablecard, and for some reason channel 690 comedy central HD just says searching for signal. But the SD channel 190 works fine, any else seen this?
That usually means that the signal on that channel is not strong enough to give you a pic. First thing to try is to disconnect the coax from the cable card and then reconnect. Sometimes re-seating the cable is all you need. Also make sure that all the coax connections (at the cable card and at the wall/splitter) are tight.
If that fails try swapping out the coax cable. could simply be a bad cable. Yes a bad cable can affect only individual or random channels.
If all else fails you may need to contact tech support to have a tech come out and check the signal strength