I think "I don't know" as a response is soooo very frustrating as it leaves you totally in limbo. However, I do believe that actually it can be the truth.
One of my biggest struggles is when I've asked, "Where was I, in your head, when you went back and did it again after having told me how much the first time reviled you?"
But I believe, for now, he didn't know. Maybe I was 'compartmentalised' at that time