The Hi-Fi Cast app relies on Android itself to discover the media on your local device and to categorize it into albums, artists, genres etc. Android does this via the Media Storage system app. See here for more details.
Try any of the following:
If you use any of the above methods you may need to refresh the Music Library in Hi-Fi Cast. Tap the 3-dot menu from he Music Library screen and select Refresh.
This is usually caused by the Android media scanner failing to extract the album art. The only way I know to fix this is to clear the Android Media Storage data. See here for details.