Welcome to the Apple Support Communities. If you are not seeing the back up being preformed properly, there are a few troubleshooting steps that you can follow in this article.
Another quick troubleshooting step is just to turn off Time Machine, then turn it back on again.
It's possible the preferences file that contains your instructions to Time Machine may be damaged. One thing you might want to try is a "full reset" of Time Machine, per #A4 in Time Machine - Troubleshooting.
If that doesn't help, see if the problem also occurs in another user account (create one via System Preferences > Users & groups if you don't have one already.