To ensure you're solving the right problems, you almost always need to perform some kind of diagnosis. It's every consultant's duty - doctors do the same for their patients. But the process, if done wrong, can also hurt your relationship with the client.
This happens more often for those who work with mid-size or large organizations.
Managers are afraid of uncovering difficult situations for which they might be blamed. They see the diagnosis process with suspicion, and believe the consultant is looking for a scapegoat - someone to be charge as guilty for all the shortcomings and problems of the department.
I've heard dozens of such stories. Employees end up omitting information or straight up lying on surveys and interviews. This makes it difficult (if not impossible) for consultants to deliver any kind of impact or results.
The solution to this, and applied by many top consulting firms, is to step down the pedestal and draw members of the client to directly participate in the process.
It could be as simple as creating a consultant-client task force to work on the survey, data analysis, or any other parts of your diagnostic process. When people participate in the process, they are more likely to acknowledge their role in problems.
Also, there are two excellent side-effect of drawing client staff to your diagnosis process.
First, it should increase your sales effectiveness. The more people are involved in the process, the more likely they are to accept changes to the initial scope of the project. One thing is hearing from you that there are larger problems to solve, another one is learning it themselves.
Second, it gives you a chance to learn your client better. You get some sense of the skills and personality of key employees - what they can do and how they work. This allows you to come up with a better recommendation or implementation plan, and something that the client will more easily accept.
If you're dealing with large clients or a team that might look at you with suspicion, remember to request their participation in the diagnosis.