Did you enter the IP address manually?
Are both primary and secondary machine connected on the same local area network?
Do you have third-party firewall software installed? Or antivirus with own firewall settings?
Do you have VPN currently active?
With Command Prompt, can you try to ping the IP address of primary machine from your secondary machine and vice versa? to check if both machines are reachable to each other..
For further analysis, could you please answer all the questions above and run dxdiag.exe on your primary machine, click "Save All Information", save the output file (dxdiag.txt) then attach it on your next reply.