Airplay Icon only shows up sometimes and doesn't have the option for mirroring.

I just downloaded the app Reflector for my macbook pro. I have double clicked the home button on my phone to get to the taskbar, I have swiped to the right twice to get to the volume bar and I have clicked the airplay button. But the only problem is there is no option for airplay to my computer. I just checked my phone and now the button is gone. What should I do, please help!