In a Hybrid deployment, lately I had experienced that:

1. On-premise users can see cloud users’ free/busy

2. Cloud users cant see on-premise users free/busy

Note: the below may not solve your problem, but it should lead you to the right way to brute force your way to solve the problem lol

Resolution:

1. Try out the Hybrid Free/Busy Troubleshooting Tool.

2. Check out: User can’t view free/busy information for a remote user in a hybrid deployment of on-premises Exchange Server and Exchange Online in Office 365

3. Did an IISRESET as recommended: http://jesperstahle.azurewebsites.net/?p=242

4. Update/Refresh the Federation Metadata. See this blog for more information.

•  Connect to Exchange Online in PoweShell
• Execute:
• Get-FederationTrust | Set-FederationTrust -RefreshMetadata

5. Execute the free/busy test from Microsoft Remote Analyzer

6. Ensure that in all Exchange Servers (including the inactive ones located in the DR sites), the Get-WebServicesVirtualDirectory has the correct ExternalURL: https://mail.contoso.com/ews/exchange.asmx (and is routable from the internet)

Get-WebServicesVirtualDirectory -Identity “ServerName\EWS (Default Web Site)”

Leave a comment if it’s still not working. No promise that I can help you fix, but I’ll try 🙂