The FBI probe is a background investigation, not a criminal probe. Kav has already undergone six of them. The new "fact" being chased down is the allegations of teenage behavior. Regarding the key charge made by Blasey, it appears that the FBI is effectively running a background check on her, for purposes of assisting the Senators to assess the credibility of the charge.
That's why Ford has not been re-interviewed. The FBI's task is merely to take what she has already said and offer evidence the may go, one way or another, to her credibility and motivations. Her apparent fear/discomfort at taking a polygraph juxtaposed with information that she coached others on how they work. Her provision of her therapist's notes to the Washington Post but not to the FBI. And so on.