Great question, Believe me, I wish I knew. I do know this though.
A person MUST be ready to hear the truth before they could possibly accept the truth.
A person needs to have their own questions first. What questions would have a believing JW question their faith in the WTS? They need doubt that what they took as rock solid has been shaken.
When ever I read about a former JW coming to their senses, i find different things shake different people.
I find that rarely is it a doctrinal problem. For example, there are so many biblical references that prove the WTS as a false prophet or at best, very very erroneous interpreters of the faith. To me, if that doesn't make one question, then what would? Then I read about some obscure, almost menial issue that makes one question. I am left scratching my head as to, "How did that little thing make them wake up?"
I think a person must be ready from their own experiences before they can recognize that they don't have the truth.
Just my humble opinion.