I was having the same problems after the Comcast-initiated "need to change your password." I went from about 140 TVE channels to about 60. I tried some of the things referenced in this thread (deleting the source, waiting 48 hours, etc) but I saw one suggestion that said we have to call Comcast customer service and they need to do something on their end.
I was able to do this via their chat system after I was able to chat with a real person. At first, there was some effort to blame my end of things (maybe my network, maybe my connection to the Xfinity broadband, etc) but none of that had changed for me.
The KEY was to make it a problem with the Xfinity Stream app not showing many Live Channels in the guide. There I had only 5 video TOGO channels showing and about 10 Stingray music channels... where formerly, it was hundreds of video channels and many Music Choice channels too.
Once I shared that, they put me on hold a few times and kept coming back with "now try it." Close the app, reopen, same. On hold, wait, "now try it". Close the app, reopen, same. On hold yet again, wait, "now try it." And after a few rounds of this, suddenly the app reloaded with what I recalled as the many channels available there.
I checked a few random ones for playback on the tablet. They played. So I thanked Xfinity chat service help and closed the chat.
Then, I go right into Channels, Settings via website browser. Added a "new source" (since I had TVE closed for another potential 48-hour wait) and attempted the TVE login again. This time was different. I could see it scanning many more channels than it had in the latest rounds of tries.
And now, I have 141 TVE channels in Channels again.
As much as I hate to suggest it, the direct solution to this problem seems to be to call or chat with Comcast until you get live help and make the problem NOT about Channels but about the mobile Xfinity Stream app not showing all the channels it used to show. Once they show there, Channels TVE will probably reload all the TVE channels too.