First, make sure you have downloaded the latest version of the app from the App Store (for iPhone and iPad) or the Google Play Store (for Android devices).
Next, make sure your device is operating on the latest version of your iOS or Android system software. Using an outdated version of the app or your device software can cause app performance issues.
Finally, try restarting your device.
If you are still having trouble after updating and re-booting your device, we recommend that you follow some troubleshooting steps from Google to try to reset your Chromecast.
Here's where to start: https://support.google.com/chromecast/troubleshooter/2995236?hl=en&ref_topic=3447927
If your Cast device isn't showing up as an available device when casting from our app, you can also try these steps: https://support.google.com/chromecast/answer/7172427?hl=en&co=GENIE.Platform