It sounds like one or more Intune endpoints are not open through your network/VPN. Check out this page https://learn.microsoft.com/en-us/mem/intune/fundamentals/intune-endpoints?tabs=north-america where you can confirm that the endpoints for Intune are open. The sync can also be initiated from the Intune side (locate the device, select Sync Settings) but if the network destination is unreachable by the client, it won't be able to sync policy. Check your VPN configuration and verify the destination addresses are reachable.