We are so trained to answer that question from a social standpoint. Socially we don't burden others with our troubles, so "Good" or "Fine" are the responses we give. And it's okay to respond that way to T also. But then go on, to talk about how you have been feeling, because the "How are you?" is an opening to that, an opening to "How have you been feeling?" or "What's been going on?"... or any other kind of opening.
It's a place to begin

My T is currently and temporarily taking notes in my session for a class she is in. (She says she's learning how to 'hear me' better. ) So last session, as we were talking about the unusually cold weather that day, she was finding her notepad and pen. When she sat down with them, we had just finished the weather chat. So there she sat, prepared, pen poised casually and looked at me. I laughed and told her I expected her to say "GO!!".