There are a couple of things I could suggest. First, if your TV has a signal strength meter, does it show a low signal strength on the channels that the TV tunes but the Tablo doesn’t? Tuners vary in ability, and it’s possible that the signal could be just below the threshold of what the Tablo needs, but the TV can tune it in. (And I know you commented about the other channel coming in ‘clearer’ on the Tablo, but I’m not sure I understand that. With digital signals, the picture is either perfect, stuttering/macroblocking, or nonexistent.)
The other thing I can suggest will be harder to diagnose, and you’ll have to deal with the station engineer to resolve it. A little bit ago there was a post here from a user who lost a couple of channels after having them for a year. It reminded me of something I had happen several years ago running a Windows Media Center setup. My 1st post is here, and then a couple of posts down I made another post with some additional info I rediscovered:
To summarize, along with the video stream, the ATSC standard also specifies additional metadata that provides additional information about the channel content. This is the Program and System Information Protocol (PSIP) data. What happened to me is one of the local stations did some upgrades to their broadcast system, and in doing so they introduced a misconfiguration in their PSIP data. Not every device is susceptible to this sort of error. My (and probably everyone else’s) TVs didn’t care and could still tune the station in, but WMC wouldn’t tune it. Specifically, there was an inconsistency in the Transport Stream Identifier (TSID) between two sections of the PSIP data.
Since I was running PC based tuners with WMP, I was able to use a PC program called TSReader Lite to read the PSIP data from one of the tuners and could show the station engineer what the issue was. Unfortunately, that’s not possible with the Tablo, at least not for us regular users. Maybe Tablo support could look at it.
The OP of the linked thread hasn’t replied back, so I don’t know if that or some other PSIP data error turned out to be the problem, or if the Tablo is even susceptible to that sort of problem like WMC was.