Check the clients GUI:
Help and support > Troubleshooting > Group
Verify that location with the SEPM.
Then check the clients logs if there are any entries:
Clients GUI > View logs > Client management.
Check for any errors or failed connection attempts.
Then back in the SEPM, Verify that it's not in user mode. Then you might want to check the heartbeat interval for that group as that client may be slow and gets repeatedly bumped off the queue during the update process.