My website is very basic and strictly content. When I created my adsense account 2 years ago, I just created one basic text ad format and I use that code on every page. I did not setup custom channels or anything.
I don't know why I never noticed this, but when I just looked at my URL Channels performance report, I don't see a lot of active pages on my site in my report. 2 of my top visited pages are nowhere to be found, but ads are on them and the code is the same as on urls that are appearing in my the URL Channels performance report. Is this common that URL's do not appear in reporting? They are not new URL's. Also, the click data in the URL Channel performance report seems incorrect...under counted.
Did you reorganize your site since creating the channels? The URL channel names are not linked to ad code. If you're serving ads at a new URL, you'll need to create a matching URL Channel name under the "My Ads" tab before you'll find it in your reporting.
I never created URL channels so I'm not sure why it shows some but not others. I was just afraid that perhaps I wasn't getting credit for clicks on pages that weren't listed in the URL channel report, but I have since created different ad units for all my pages and now I have much better visibility and accurate reporting.