It is true that generally, Subject Matter Experts do not lend themselves easily to the conduct of a Training Needs Analysis.
Keep in mind that SMEs are not necessarily formal educators and thus, may not fully understand the need for such analysis. By the very nature of the TNA, the instructional designer must break down the subject matter down to its most basic elements. Being highly experienced members of the target population, SMEs often find such exercise boring and to some extent, pointless.
However, their implication is essential for the accuracy and integrity of the results. Furthermore, their input will be required throughout the development process, and will become vital during the client acceptance phase. So it is worth taking the time to create a positive rapport with them at the very beginning.