HI Davide, the users themselves have to sign into their account and run their migration themselves before an admin account will be able to see them.
Unfortunately I don't think there's a way to migrate certs for all users as an admin (If i'm wrong someone please correct me). We had this issue as well with a lot of learners / instructors asking where their certificates went.
https://gist.github.com/CrawdadWarden/eaa8756cff1ea06ee827c715f3e0cb0f I wrote a script (which is mostly the user based migration code from the 5.4 repo) to migrate all of the users' certificates. I'll pop it here as is if it's any help to you. I'd take proper backups etc first if you do decide to use it.
Thanks,
EVan