Gradarius can offer the best feedback when you take small steps. If you aren't sure what you did wrong, try entering detailed steps when you rewrite the previous cell.
Additionally, the articles below can help you improve writing your solutions in Gradarius, which in return will also help you improve the feedback you receive from Gradarius.
It is highly recommended that you complete your first tutorial assignment before you read the following articles.
The difference between Tutor Mode and Solo Mode (aka Exam mode), as well as the various kind of feedback Gradarius can give you about your work and how can you use the feedback to improve your solution to get full credit for your solution.