You can't just keep taking new lessons. You have to do "strengthen skills" or go back to practice older lessons on a regular basis. In fact, the new scoring system they just released with levels (crowns) for each lesson is supposed to emphasize this; to really complete something, you have to go back and practice it many times.

When I do duolingo, I usually try to do about a two to one ratio of strengthening skills or going back to older lessons with taking new lessons.