Can you tell us your machine info:
- Primary machine operating system (e.g. Windows 8.1 / 10)
- Secondary machine operating system (e.g Android/iOS/Windows/HTML5)
- Primary machine's Graphics adapter (e.g. AMD/Intel/Nvidia/Microsoft Basic Display)
- Network connection type (Wired/Wireless/USB tethering/Wifi Direct/Hosted Network)
Are both primary and secondary machine connected on the same local area network?
Do you have third-party firewall software?
Do you maybe have VPN active?
For assistance, please also check Troubleshooting - Network connection chapter in user manual https://spacedesk.net/user-manual/.