You cannot fail, because it is not a test! If you break the law by entering controlled airspace, or display poor airmanship, you are quite clearly in need of further training before anyone will give you a licence. It follows, that an unsatisfactory cross country should be repeated or you may not pass the navigation section of the Skill Test. I have failed 5 candidates on Nav in the past year, in each case, it was because they had not be taught to navigate properly.