Hi, @create share
Thank you for posting in Microsoft Q&A forum.
Green tick means the device is online, grey X means it's offline. Clients send a ping to their management point every 5 minutes. If the management point stops getting pings the device gets a grey X.
We may start from checking the CcmNotificationAgent.log and CCMMessaging.log on clients and BGB logs on MP server.
You may check this article for the details:
https://www.anoopcnair.com/overview-client-online-status-sccm-console/
(Please Note: Since the website is not hosted by Microsoft, just for your reference.)
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