I recently bought a digital TV - Sony XBR9. I have basic Comcast and they just switched to digital in my area. My TV can find lots of HD channels (9.1, 9.2 etc.) and I have many more channels than I used to. But all the non-HD channels (4,5,6,7,8,9,10,11,12,13) are missing. You might ask, why would you want them if you have the HD versions? Well, when I want to go straight to a channel I want to hit just "5" on the remote, not type in "5.1". So how do I get those channels? If I can't, how do I navigate to the channels I want with one button on the remote?
It sounds like you have no box and are directly connected to your TV. You may have the non-HD's, but they may have odd channel numbers. Also, when Comcast goes digital, that means all of the expanded basic channels like Discovery, History, etc... will all be encrypted. You will only be able to get basic cable (broadcast and public access.) And that will probably not last long as the FCC has said cable companies can now encrypt EVERYTHING.
I have both a Sony and Samsung TV. It's just the way the Sony works. On my Samsung, when I've tested it directly (I normally have a DTA on that one), it's smart enough to know there is no '5' and will jump to 5.1 (if I wanted analog 5, I would use 5.0). On the Sony, it will try direct-tuning that channel's frequency if you don't type in a known number.