Here are a couple of things to try.
1. Reboot your Roku, and your Router by unplugging your Roku first, then power cycle your Router. Once it's back up, then apply power to your Roku.
2. Also verify that your Roku and your phone or tablet you are casting from are all on the same network.
3. Finally, if the first two steps did not fix your mirroring problem, also try restarting the device you are using AirPlay on as well. By powering it down and then turning it back on.
Roku is my favorite streamer, mkvXstream.com has my favorite Roku channels