I feel like I understand the concept, why I am I still not getting full marks on the question?

Understanding concepts in exams is only part of the battle. To improve your marks you need to improve your exam technique!

For example: Have you correctly interpreted what the question is asking of you? Is your writing concise and to the point? Have you taken account of how many marks the question is worth? Do you understand that most questions require application of your understanding rather than just regurgitation? If the question is split into parts have you allocated your time appropriately? Have you re-read your answer critically?