Dell is extremely strict and incredibly complex requirements on when and how often warranty information can be polled for a given host. Without going into specific details the "how often" is related to how soon the server is due to expire. Without understanding all the ins and outs of how warranty status polling works, it's sufficient to say that in all likelihood SAM hasn't yet polled Dell's servers for warranty information for these particular hosts yet. You can wait a few days to see if this updates on its own, or you force SAM to query this information by using the "Poll Now" button located in the "System Information" resource of the "Asset Inventory" tab shown below. It may take as long as 15min or so for this to update in some cases.
