Some apps won’t Screen Mirror, how is that possible?
There is a simple test that you can do to see if the app that you want to mirror has blocked Screen Mirroring:
Open that particular app and slide up Control Center on your iPhone/iPad. Then, click on Screen Recording and start recording the screen to your Camera Roll. Watch the app that you want to mirror and at some point stop the screen recording. Then go to your Photos app and watch the video that was recorded. If the recorded video shows the app you want to mirror, it works fine. If the video is empty, then the app you want to mirror does not allow Screen Mirroring.