It happens in two scenarios : 1) You resubmit a solution to one of your previously "pretest passed" submissions, in that case only the last submitted solution will be judged and all prior submissions will be skipped.

2) You submit only once, but your submission matches exactly with someone's code (completely or to a great extent), which implies that you cheated(most likely), and hence as a lighter action, your submission will not be graded.