I had the same experience of not being able to view certain issues, for example Popular Science August 2012, Readers Digest Canada Aug 2012, Popular Science Nov 2012.
I finally got it solved last night, and was able to download and read all those issues that didn't work before:
1. On the TP, delete the Zinio app.
2. Connect TP to PC, USB drive mode.
3. Using File Explorer on PC, delete the entire zinio folder on the TP.
4. Eject, disconnect, and shutdown/restart TP.
5. When TP restarts, redownload the Zinio app, log in to your account, and now try to view those problem issues.
Fixed! At least it worked for me, and I can now enjoy those issues that I couldn't read before.