Hello @Donald Fraser ,
If it happens uniquely to Dell computers from newer series, I would recommend to post your question in Dell Forums, since their experience would be more focused to factors involved those devices. I would suspect of some pre-installed software or BIOS settings affecting the discovery.
What I would try in this case besides the SMB1 and IPv6, would be to enable the remote desktop feature for this computers:
From Control Panel, select System
Then select Remote Settings from the left panel
Got to Remote tab and enable Allow remote connections to this computer.
Ok and restart.
Hope this helps with your query,
Best regards,