How did you transfer from the S9 to the S25? The best way to transfer is by using Smart Switch.
If you use Google Calendar, make sure all the google accounts from the S9 are also on the S25, and make sure the sync is on for google calendars.
If you go to Settings >> Accounts and backup >> Manage accounts, make sure the accounts which are on the S9 are also on the S25. This includes not-Google accounts too in case you are using their calendars.
For each account, click on the email address >> Sync account and make sure the toggles are on for calendar (best to make all toggles for every account match what is on the S9).
Also, to force the calendars to sync, you can toggle the switch next to calendar off and back on, and see if that helps.
