You're missing a key point. That everything you 'know' you comprehend at a certain level. Someone could be very knowledgeable, but still not that smart.
OK so you mean that the person himself can't grasp. I thought it's me who can't grasp.
To make these people comprehend, you can try explaining with the help of real life examples as it's going to be very tough. I have tried taking tuitions of such students and it was really tough to make them understand what I am saying and so I made use of examples which made my job easier.
If you are having a general conversation, then I have no clue as whatever I would tell them, they wouldn't understand it or may just pretend not to understand.