Are you using macOS 15.x Sequoia? There were many reports about device discovery issues on the Roon forum up to 15.2, which required fiddling with the network permissions.
However, there have now been reports that this might be fixed in Apple’s new update to 15.3, as released yesterday. It seems that macOS after the update now finally shows a message, asking you to grant network access permissions to Roon. Approve this and hopefully it should be fine.
Let us know if this doesn’t help, so that the forum hive mind gets some more information on whether 15.3 really fixes it or not.
If it does not help, or if you don’t want to upgrade macOS right now, try going to the macOS Settings > Privacy and Security > Local Network. There should be two toggles for Roon and Roon Server. They must be enabled and most likely already are, but nevertheless toggle them off and then back on again. Maybe this helps already. If not, try rebooting the Mac. This normally helps, but might have to be repeated if you update Roon or reboot the Mac. (Hopefully the 15.3 update should fix all this)