Well, you need to let them know that unless they are not interested in keep improving and feeling better, measurements are in place for a reason and it’s the best way to record their progress and that it helps you with updating and customizing their training program. It’s as simple as that. Of course, you can’t force any one to participate in this policy/program if they don’t want to…it’s their loss.
am I correct in assuming that the measurement updates include a tape measure wrapped around those reluctant clients? Could you possibly ask the clients that they have their measurements taken at home by a friend or relative?
While I see value in measurements as they express themselves in numbers on a scale or inches, it is also a very superficial way of assessing people. They may have agreed to it when they first joined but probably did not enjoy it all that much even then. And they are also reluctant because they probably have little reason to expect improved measurements, else they would agree to it.
Also consider that improvements in fitness can happen even if they do not show up in the measurements you are after. Improvements in some medical data such as fasting glucose, blood pressure and cholesterol are even more important, not to mention improved self efficacy which cannot be caught with a tape. Insisting on assessing progress as you describe may end up un-motivating your clients when they see you come with the tape measure.