I agree with many points that Mike has mentioned already. Some clients may be uncomfortable with certain measurements, and there are other ways to acknowledge their progress. Weight is an extremely sensitive issue, and building initial rapport and trust may take some time. You can certainly explain why you would like to keep track and inform your client of the connection between the numbers and health. However, your client needs to feel that connection with you and then, maybe, over time they will allow you to track progress. If not, then I would respect their decision.
Keep on encouraging and motivating your clients!