A single Certificate can not issue multiple certificates to a single user. Thus your people are probably seeing their original certificate, not a new one. Ran into the same problem and have a workaround.
I assume that the big picture is that a person needs a certificate, at least once a year. The certifying agency does not care if someone got a certificate in January, and another one in June. The clock always starts ticking with their last certificate, when ever they earned it.
I pick a date and create a new certificate and qualifier every 12 months ( say the fiscal or calendar year end). It is the same course. I hide the old certificate and old qualifier. The certificates reads exactly the same (I get cute and change the border color) but only the issue date and the unique code change. We use a quiz qualifier to determine if someone should get a certificate. I duplicate the quiz and link the new certificate to it.
I encourage and tell users how to look up their issued certificates in Moodle to check the date on their previous certificate. It does mean I have to download and combine files (current and previous issued certificates) to see whose certification has lapsed. But it solves the problem.
I considered other options like conditional activities or creating a new course for a new cycle, but for us, my solution worked. As the certifying agency, I need the history of the qualifier, so I did not check out if I deleted all attempts at a quiz, would that mean the user's record of having passed it would be recognized by certificate? Create a new certificate and new qualifier in the same course was the easy way for us.
Hope that helps.