You may be creating an email campaign within which you are inserting a link through to an Update Form?

A: This function is only fully operational when your email is sent out fully, not when sending a test (preview).

Hint: to use the correct Update Form tag, remember to use the clickable link at the top of your screen: 

"For information on how to personalise your email, click here to look at the tag list! "

and when creating the link to your Update Form firstly create some clickable text, highlight that text, click on the Link icon (chain link), select protocol other, paste your tag into the url box and click ok. (an example of an update form tag could be %updateform27%)