Hybrid Free Busy Issue

It can be several things but I usually recommend reading these articles first to make sure you have a supported design:
https://techcommunity.microsoft.com/t5/exchange-team-blog/the-hybrid-mesh/ba-p/605910
and
https://techcommunity.microsoft.com/t5/exchange-team-blog/demystifying-hybrid-free-busy-what-are-the-moving-parts/ba-p/607704
and
https://techcommunity.microsoft.com/t5/exchange-team-blog/demystifying-hybrid-free-busy-finding-errors-and-troubleshooting/ba-p/607727

I recommend running the tests on this website as it will often give you a helpful error message to research:
https://testconnectivity.microsoft.com/tests/o365

This article has saved me several times.
https://docs.microsoft.com/en-US/exchange/troubleshoot/calendars/cannot-view-freebusy-information-for-remote-user

Once I had to refresh the metadata with this command:
Get-FederationTrust | Set-FederationTrust -RefreshMetadata

These commands are helpful to run in the on-premises Exchange Management Shell:
Test-FederationTrust and Test-OrganizationRelationship
See what error messages you get.