The level you have achieved in a skill is related to the number of correct answers you’ve given (and the number of pix earned in that skill).
If you can't resume a test, it means that you have reached the farthest you could go.
To try to improve your score, you can try again the questions you didn't answer correctly by clicking on the "Try again" button that appears on your profile after 4 days; or you can start the skill again from scratch by clicking on "reset".
Moreover, we are still in the process of creating the levels 7 and 8. This means that the farthest you can currently reach is level 6 in each skill and a total score of 768 pix (out of 1024).