If Foobar2000 Copilot is having trouble connecting to your Foobar2000 server PC, make sure:
1) Your phone's Wi-Fi is ON and your phone is connected to your local area network.
2) Foobar2000 is running on your server PC and that PC is also connected to your local area network.
3) The Control Server (foo_controlserver) component is installed in Foobar2000 on your server PC. This component is what allows Foobar2000 Copilot and Foobar2000 to communicate over your network.
Make sure its default settings are set. On that component's preferences page in Foobar2000, click on the 'Reset page' button, and then the 'Apply' button. Do NOT press the 'Reset All' button, since that will reset all your Foobar2000 installed components back to their defaults.
If you don't have this component installed in Foobar2000, see : foo_controlserver plug-in install.
4) The IP and Port address you set in your Foobar2000 Copilot server definition match the IP and port specified on your Control Server preferences' screen on the Foobar2000 PC server.
5) Windows Firewall is not blocking Foobar2000. If you have checked steps 1-4 and Foobar2000 Copilot is still not connecting, check the Windows firewall settings on your PC to see Windows isn't blocking access to Foobar2000.
See: Check Windows Firewall settings to make sure communications to Foobar2000 is not blocked
Reserving IP Addresses on your Router for your Foobar2000 server PC
You likely are using dynamic IPs on your router and letting it assign those to the various Internet devices on your network.
The issue is unless your reserve addresses on the router, it may dynamically assign different local IP addresses to your Foobar2000 server PC at different times -- so the IP addresses you have saved in Foobar2000 Copilot may initially work and then not work if the router re-assigns those addresses.
The particulars on how to reserve IP addresses depends on your specific router. Look under your router documentation. By reserving IPs on your local network, you can guarantee your Foobar2000 Server PCs will always have the same IP addresses on your local network.
If, however, you choose not to reserve router IP's and Foobar2000 Copilot stops communicating with your Foobar2000 server, do a quick check of the IP address assigned to your Foobar2000 server PC (see: Determining foobar2000 server IP). If it has changed, update your server settings in Foobar2000 Copilot.
Turn on the 'Keep Wi-Fi on when the screen times out' option if running on an older WP8.0 phone
Skip this if you are not running WP8.0.
Another recommendation (not a requirement) is that on your Windows Phone under Settings>Wi-Fi>Advanced, you turn on the 'Keep Wi-Fi on when the screen times out' option.
You may not have this option on your phone. For Lumia devices, this option was available under WP8.0, but has been removed on WP8.1, where the system appears to manage Wi-Fi more efficiently.
The 'Advanced' button is at the bottom of the Wi-Fi Settings screen. If you don't check that option under WP8.0, your Windows phone can by default power down the Wi-Fi antenna to conserve the battery when the screen times out.
But when you reactivate the screen and Foobar2000 Copilot resumes, the phone will have to re-enable the Wi-Fi antenna and re-connect to your router, which can take a few seconds. You will see the phone's Wi-Fi connection icon at the top of the screen 'strobe' until it's fully on.
Foobar2000 Copilot during that time will display 'Waiting for Wi-Fi...' until the antenna powers back up and the phone has reconnected to the router.
Also throughout Foobar2000 Copilot should you try to perform a function requiring a connection to the Foobar2000 server and the connection is down, you will see a visual indication of this on the screen, typically a message flashing 'no connection'