There should be a practice session where you get all the questions you've gotten wrong in the past. The way it is now, you can continue progressing with some gaps in your knowledge.
We keep track of when you miss words and when you're particularly weak in a certain concept. Word strength decays over time (as you can see by the gold bar on each skill). This makes it possible to go back and practice what we know are your weakest areas. You can practice here: http://www.duolingo.com/practice If you'd like to practice your weakest words go to the vocab page here and select 'practice weakest words': http://www.duolingo.com/practice