I mean, there could be something in your energy blocking out the light.... if that makes sense? When you put something in front of the sun it blocks it out and all you see are shadows instead... like with the solar eclipse :) So, we all generate our own inner light, which is what you see in the aura, the light radiating outwards. But if there's a blockage then you won't be able to see anything, instead it can look grey, black, or maybe just empty as if nothing exists there.
Blockages are usually beliefs we have about ourselves, which say things like "I am not worthy of this light"... (as a very broad example), so the belief being a tangible thought-form will block out that light. Considering you're having trouble seeing around your head, which is crown chakra area, it could be a belief as simple as "I am not connected to my divinity"... though what is strange is that you can see it around others and yet most people don't have open crown chakras.
I don't really know... it's just a possibility. :) Are you a good visualiser? You can use your mind's eye to look too, see if you get anything then. :)