This question is similar to one I just posted an answer to, so I am pasting it here as well.
This sounds like an opportunity to change client perspective. I would approach the client as cheerfully as I could (some times you have to fake it a bit, but if you put in a sincere effort you will end up cheerful anyway). Something like “Hi Donna (using their name is very important, as is a big smile)!! I just noticed that it is time to update your measurements. I would be happy to help you with that. (If they get grumpy) If you don’t feel like it right now, I understand. But this is a really good time for me to help. It will only take a few minutes. (Smile and wait for a response. It is important to keep smiling and waiting.) If they continue to resist, ask when they would rather update their measurements. (Again smile and wiat.)
And remember you can’t reach everyone on the first attempt. So, try again later and often. (If at first you don’t succeed, try and try again.) On subsequent attempts I would initiate with something like, “I know measurements are not your favorite part of changing your fitness, but they can be useful to your achieving your goals (Smiling). I am willing if you are. (Smile and wait.)