% mastered reflects your long term retention of a word or phrase.

The way to increase it is by learning words that are "Ready for Review." Words will appear as "Ready for Review" on your homepage.

This means that you can't increase % mastered more quickly by learning a piece of content over and over. 

If your % mastered doesn't seem to be going up, you're not doing anything wrong. It takes a very long time to truly master a word. 

Please take a look at this article, I think it will bring some clarity into the situation: