To check for updates for the Contact Transfer Android app, on your Android, open Google Play Store. Tap the menu icon > My apps & games > tap the "Update" button next to Contact Transfer.
Disable Mobile Data on your Android, as this can interfere with the WiFi connection. Mobile data can be turned on again after using Contact Transfer if you wish.
Ensure that the Contact Transfer app is active on your Android's screen. Do not close the app or switch to another app.
Make sure that both your computer and your Android device are connected to the same WiFi network.
To check if your phone and PC are on the same Wi-Fi, make sure that your phone's IP address is the same as the IP address of your PC (only the first 3 parts should match - the last part is your device's unique identifier). The IP address is an identification number assigned to each device connected to the Internet.
Your PC's IP address will be displayed under the QR code in Contact Transfer.
Your Android phone's IP address can be found at the bottom of the welcome screen of the Contact Tranfer Android app when you launch it on your device.
If your PC has multiple internet connections, e.g. WiFi and ethernet, choose the IP address under the QR code that matches your phone's IP address by clicking on it as shown below.
Check that Windows Firewall is not blocking the connection with Contact Transfer.
Finally, sometimes all is required is a restart of your PC and/or your Android. Safely restart your devices, then try again.