Firstly, identify your issue by testing the following scenario.
- Can on-prem users view F/B information for a cloud users?
- Can cloud users view F/B information for on-prem users?
- Can F/B be viewed between on-prem users?
- Can F/B be viewed between cloud users?
- If you a running a mixed on-prem Exchange environment then also check availability between your mixed on-prem organisation and Exchange online.