Hello,
you need to click edit after adding element then save the pdf and then generate the certificate to check the changes.
if you are using any cache plugin, it might be possible that your page is cached and showing an outdated copy of the certificate.
I would like to request temporary access (wp-admin URL and credentials) in a private reply to your site to take a better look at the issue. The information you will enter is private which means only you and I can see and have access to it.