First, make sure both the Apple TV and the iPad are on the same wireless network.
If you have a guest network set up on your wifi, if either the ATV or iPad are connected to that, no sharing is allowed through a guest network. So make sure you're connected to your main network.
On the iPad, under Settings > General > turn off Bluetooth. Then toggle wifi off and then on, or power your iPad off and then on.
If it still doesn't show up, power your Apple TV and wireless router off for a couple of minutes and then back on.
If still no icon, check if your router has a built- in firewall, and turn it off or change it to minimum security. If you're not sure how to do that, ask the router manufacturer or your internet service provider if they supplied it to you.
Lastly, it may be a port issue on the router. Article HT2463 states the ports used by Apple TV and Airplay/Home Sharing. You may need to ask the same internet service provider or wireless router manufacturer how to open these ports.
Hope this helps.