I don't think they become "expired". I think that some might get frustrated and feel like their not of any further help to the client. Perhaps some might burn out the way see in other professions. I don't think that is the case with all therapists. Like in many professions there are some better than others. Unfortunately I have experienced the same as you but I've only had one therapist but have been in communication with others. I think my boys therapist had come along and is better "in tune" now. There always material to discuss but it's wether the client brings it up and how effective it's addressed in session.