Verify that the Guest Configuration Arc Service (GCArcService) service is running. Also, if you have a monitoring agent extension deployed you can check the logs on the resource blade. The heartbeat table should display activity if the agent(s) are online.
ARC and the monitoring extensions also need cloud connectivity:
https://learn.microsoft.com/en-us/azure/cloud-adoption-framework/scenarios/hybrid/arc-enabled-servers/eslz-arc-servers-connectivity