Why didn't I get a high mark on my essay entitled 'Describe and evaluate explanations of conformity' when I spoke about compliance, identification and internalisation.

This is a common mistake to make when questions ask you about explanations for conformity. The reason why you did not score very highly is because, although the terms you described are relevant to the topic of conformity, they are just types of conformity or different ways and levels that people conform on. Therefore they do not explain why a person would conform. Here the question is looking for you to show your knowledge of NSI (Normative social influence) which is predominantly used to explain compliance and also ISI (Informational social influence) which is used to explain internalisation. You can still mention the three types however it is best to explain NSI and ISI mostly and use the types to further develop your answer. Don't forget the evaluation too!