Chromecasting to streaming device.
iPhone not casting to streaming device. Doesn’t appear on app.
iPhone 5s
It's unclear what screencasting device you use with the iPhone and what is the app you are having the issue. In most cases, you need to make sure iPhone and the screencasting device must be on the same network and the app should be allowed to search for other devices on the network. If this is done, see if the issue is affecting specific app only. If yes, contact the app developer for more help: How to contact an app developer - Apple Support
If all apps are affected, contact the screencasting device support for further help.