Hi DocZ -
The troubleshooting guide for Airplay below may help you resolve your issue:
Troubleshooting AirPlay and AirPlay Mirroring
If the AirPlay icon doesn't appear
Check for the AirPlay icon ( ) in the recently used apps section on your iOS device, or in iTunes on your computer (as in these articles about how to use AirPlay and how to use AirPlay Mirroring). The icon will only appear if your device or computer is logged onto a Wi-Fi network which has an AirPlay-enabled recipient, such as Apple TV (2nd generation or 3rd generation).
If you are still unable to see the AirPlay icon, try one of the following steps:
Verify that your AirPlay-enabled devices have the most up-to-date software or firmware.
Verify that your iOS device has Wi-Fi turned on. Enable Wi-Fi on your iOS device by going to Settings > Wi-Fi.
Verify that all your AirPlay-enabled devices are logged on to the same home Wi-Fi network (check the home network names on your devices to confirm).
If trying to AirPlay, or AirPlay mirror, to your Apple TV (2nd or 3rd generation), ensure that AirPlay is enabled on your Apple TV as well. You can enable or disable AirPlay on Apple TV in the AirPlay menu: Settings > AirPlay
Check Internet or network connectivity on all affected devices.
Temporarily disable firewalls (both network and local) and security software. Security software and firewalls may block ports that AirPlay uses to stream content. You can often resolve issues with security software by updating, correctly configuring, or uninstalling security software.
If attempting to use AirPlay from a third-party app or a website from your Safari app on your iOS device, confirm that the app or website is AirPlay compatible (refer to the developers of the app or website for additional information).
Troubleshooting performance issues with AirPlay or AirPlay Mirroring
If you are experiencing intermittent playback or significant network lag with AirPlay or AirPlay Mirroring, it could be due to a weak Wi-Fi connection, interference, or the distance between the Wi-Fi router and your iOS device, Apple TV or AirPort Express. Try the following suggestions:
Ensure that other devices are not trying to stream to the same Apple TV at the same time.
Turn off Bluetooth on your iOS device by tapping Settings > General > Bluetooth.
Ensure that your Wi-Fi router is set up with the recommended settings for the best performance.
Certain external devices, such as microwave ovens and baby monitors, may interfere with a Wi-Fi network. Try moving or disabling these devices.
If possible, try to locate your Wi-Fi router in the same room as your Apple TV and iPhone/iPad.
If your wireless and wired networks are the same, try connecting your Apple TV to the router via Ethernet instead of Wi-Fi.
If the Wi-Fi router has an external antenna, check to see that is it connected properly and in good condition.
Use the Wi-Fi network troubleshooting guide to resolve interference and other issues.
Hope this information helps!
Same problem as I have in my thread here:
This seems to be a common problem, with no known fix that I can find. Rebooting the phone or wifi AP or Apple TV fixes it for a day or so, but the problem alway returns.