First, make sure that your sending device and your receiving device running AirServer are on the same network.
If you are still unable to discover AirServer, make sure that your firewall or Antivirus software has an exception for AirServer.exe so it doesn't get blocked when trying to broadcast across the network.
How to allow software through the Windows Firewall
If AirServer.exe is already allowed through the Firewall or Antivirus software, try holding down the home and lock button on your iOS device until the device has restarted. Sometimes the iOS devices go into a state where they are holding on to old data which prevents them from discovering AirPlay destinations. A restart from the device using the "Home + Lock Screen" option should clear the cache on the iOS device and allow you to discover AirServer.
If you are behind an advanced network such as a Corporate or University network, it's highly likely that Bonjour is not being broadcast on the network which is required for discovery to work.
When dealing with multi-subnet environments, VLANs, or Layer 3 networks, you should first ensure that multi-casting is enabled.
Bonjour will need to be accessible across the whole network. Bonjour issues between networks are common when dealing with these environments. In most cases, Bonjour will not be accessible across the network, as it is blocked by default.
Here is a guide you can look over, or send to your network administrator, regarding Bonjour across Layer 3 Networks and Multiple Subnets:
Wireless LAN Apple Bonjour Deployment Guide by Cisco
If your Wi-Fi router supports interface isolation or Wi-Fi isolation, make sure that these settings are turned off. These settings prevent wireless devices from communicating with each other. Refer to the documentation that came with your Wi-Fi router to access its settings and make sure your router has the latest firmware. Additionally, you could try creating an ad hoc network on your computer and connecting your iOS device directly to this signal. If the connection works with an ad hoc network, the issue is most likely with your router.