The best course of action, to begin with, is an informal discussion with the person involved (or module leader) during their office hours as to why the piece of work has got the grade it was given. It really isn't possible for lengthy written feedback in many cases but any half decent member of teaching staff should be willing to discuss a piece of work further at the appropriate time. If you are not happy with the outcome of that it should be possible to raise the issue with the head of department but you may well find it not to be necessary once you've had a conversation and gained insight.