The simple answer--God changed his mind with Hezekiah. I also believe there are other instances in the bible where God changed his mind due to the repentance of the people.
Also with the JW's, they currently and in the past made false prophecies that didn't come true. The BAC's that I know have never made any prophecies tying the end to a specific generation of people. They have always made vague references, and with good reason to--Jesus said no one knows the day or the hour.