Hi! Nobody will be able to give you an answer here, as there could be hundreds of reasons. You should let a competent person run a diagnostic on you to see where you stand and what kind of work is needed to become "offer-ready". Based on this, you can create a serious plan (which might include to postpone your interview!).
Just make sure that you are talking to someone who has conducted interviews at McKinsey (just having some McKinsey consultant give you a case is not enough unfortunately! The person must have been trained on interviewing by the Firm to really understand what is actually being tested, and how!).