Chromecast and the device from which you are posting content must be on the same WiFi network. I have spent some time I had the Wi-Fi turned off on the iPhone and did not show me the YouTube icon Chromecast until I realized the problem and returned to turn Wi-Fi. At other times he had more than one WiFi and had the tablet on a network and another Chromecast logically could not send content to wearing the same network.
There may be more reasons to not see the icon Chromecast in an application. Sometimes problems can be solved passengers restarting the application, smartphone or tablet, and even Chromecast router. Other times, it will play investigate further to discover the real problem.