You had talked about a JAD sessions. Were you comfortable just attending them or did you facilitate them?
See usually one approach that we followed of finding use cases was by interviewing the potential users of the system. (There can be ambiguity in this case)
So we have JAD sessions. We had sessions called Joint requirements planning workshops (JRP).
These workshop pulled people together a group of people interested in the system being developed (basically the stake holders).
Everyone one in the group was invited to give their view of what the system needs to do. And yah Key to success of these workshops is the facilitator.
Yeah as a facilitator I used to lead the group, ensuring the discussion sticks to the point and all stake holders are encouraged to put their views across.
A scribe is also present, ensuring everything is documented.
A simple method of attacking a workshop is
(i) Brainstorming all possible actors first
(ii) Brainstorming all possible use cases
(iii) Justifying each use case by a simple one paragraph document
(iv) Capture them on the model.
Again Conceptual modeling (Domain modeling) is the activity of finding out which concepts are important to our system. Also you must read our another topic on How to gather requirements in the capacity of a Business Analyst.