I noticed a post on Matt Goyer's blog today that I wanted to post here as well to help improve discoverability. Like him, I hear from customers via the contact form on my blog fairly frequently, and one of the common issues I hear is that one of the high-definition (HD or ATSC) channels that has been working correctly suddenly stopped tuning correctly on a user's Media Center.
One of the folks on the TV team explained the underlying cause of most of these issues to me:
Typically the reason this happens is that a broadcaster starts broadcasting on some other frequency, and Media Center no longer receives the signal. Media Center uses a tuning table that is supplied by our metadata provider (which they get directly from the FCC), to know which channels are broadcast on which frequencies.
When broadcasters change frequencies (either because they switch from a test feed to a live production feed or for some other reason), Media Center has no way to know about it unless the FCC knows and updates the database that the metadata provider gets its data from. Media Center does not perform exhaustive channel scanning the way that a TV, or some other TV applications, do.
There is a web page where you can report tuning issues like this - the Electronic Program Guide (EPG) issue submission form. I encourage you to report issues that you see via this form as soon as you find them to help speed up the turnaround time for providing updated guide/channel data for your Media Center system. If you send issues to me, I can try to help, but in most cases the information ends up going into this same submission form.