Under the SCCM Resource Explorer for a client, you may have no inventory data under the Hardware node.
If it is not a new client, this data should be populated.
SCCM uses this information to correctly add machines to collections based on database queries.
If there is no data, then SCCM can't manage the machine appropriately.
To confirm the issue, look in the SCCM Client logs on the affected machine in "InventoryAgent.log".
If you see "Inventory: (87d00224) Invalid message element (ReportType)", then there is an issue with SCCM Client Settings Policy being applied to the client.
In order to fix this issue:
You should see the re-mediated policy apply and the inventory run as expected.
After a few minutes, check the Resource Explorer against the client to see the data populated.