Any explanation why I still experience pain when I touch/tap on my CEREC dental crown that was placed over 2 months ago?

I had a crown and root canal done on tooth #28 on April 2015. After a week I went back because of pain when I chew and touch/tap on the crown. Bite was adjusted and xray did not reveal any problems. Went back in June since pain did not go away. Dentist evaluated it and couldn't find any problem. So he offered to redo the root canal and place a new crown. Procedure was redone on July 2015 and still now experience the same thing. What could the cause be? The dentist can't seem to figure it out.