The fact that the head of the mainsail is 5 inches below the masthead is not necessarily any cause for concern. It sounds like you are describing horizontal wrinkles in the luff, which would mean the sail is not hauled up as much as it should be.
I would securely attach a light line to the main halyard shackle, then haul the halyard (without the sail) up to the masthead. Does it still stop in the same spot as when hoisting the main? If yes, then you need to investigate why. If it goes up to the top, then something is not allowing the main to be hoisted up the last few inches it needs to be. Since you apparently have a simple boltrope setup, I''m not sure what that could be.
DelmarRey''s suggestion might be the answer. Good luck.