Hi,
Thanks for posting your question in the Microsoft Q&A forum.
1.Expect the timing issue, it is also possible that you lack permissions:
To access to the Content search tool in the compliance portal (to run searches and preview results and export results), an administrator, compliance officer, or eDiscovery manager must be a member of the eDiscovery Manager role group in the compliance portal. For more information, see Assign eDiscovery permissions.
2.Is the returned item email? For mailbox items, only email messages are available to preview. Preview for items like tasks, calendar items, and contacts aren't available.
3.I suggest search another message which you have confirmed to see if this problem persists.
4.If you cannot view the sample, you could try if you can export the results.