Yeah I know what you mean. It happens to me fairly often that a session ends at a very unsatisfying place where I feel like we're just getting somewhere interesting and then the next week I'm either not in the mood or I forget exactly what part of the topic was interesting or just feel like something else is more urgent.
I also feel like "we'll need to talk some more about this" gives me a false sense of security especially because T isn't often directive about what we talk about and sometimes I have a hard time bringing things up again. Sometimes she forgets details that I wish she'd remember and I feel upset that she's forgotten. On the other hand, this minimizes the feeling of embarrassment in having to talk about the same thing over and over!
Seriously though, I don't see how anyone could get anything done by just talking about an issue once. If something is a serious issue for you, you just need to keep revisiting it... until eventually you don't. At least for a while. I've found that there are things I just need to refer to from time to time now. But for a while they were all I could talk about.