Media Storage is a system app (ie. you don't see it in the normal apps list) that runs in the background and maintains a database of the media on your device. Media Storage is installed by default on all Android phones and tablets.
The Hi-Fi Cast app relies on the database generated by Media Storage to display the lists of Albums, Artists, Genres etc that you see when browsing music on your Android device (ie. when you have This Device selected as your media server).
Unfortunately the Media Storage app doesn't allow for any configuration or have any functionality to help fix problems (the most common being missing albums or missing album art).
You can confirm if the problem you are having is caused by Media Storage by opening another music player that uses Media Storage (eg BubbleUPnP) and see if that also has the same problem.
Many music apps will maintain their own database instead of relying on Android Media Storage.
Missing albums/songs
Forcing Media Storage to rescan your media may suffice (see below). Otherwise you may need to clear the Media Storage data (see below).
Duplicated albums/songs
You will probably need to clear the Media Storage data (see below).
The only way I know of to get Media Storage to extract album art is to clear the Media Storage data (see below).
You can try the following:
Another option is to use a media scanner app. An example is Media.Re.Scan . There are others available in the Play Store (these apps have no affiliation to Hi-Fi Cast. Use at your own risk).
If all else fails , clearing the Media Storage data may be required (see below).
To see the newly scanned content in the Hi-Fi Cast app, tap 'Refresh' in the 3-dot menu on the Music Library screen.
Please be aware that the following procedure may delete any playlists you have created with other apps that also use Media Storage.
You can clear the Media Storage data by going into Android Settings, then select the Application Manager and find the entry for the Media Storage app (you'll probably need to tap 'Show system apps' from the toolbar menu to see the Media Storage app - or on some versions of Android select the 'ALL' apps tab). Once you've found the Media Store app, tap it to see the app details. From here you need to tap the 'Clear Data' button (one some versions of Android you will have to tap 'Storage' first before you see the 'Clear Data' button).
Once you've deleted the Media Storage data you need to convince it to re-scan your media. Unmounting and re-inserting your SD card (and/or restarting your device) should do the trick. The only way I know of to monitor the progress of the re-scan is to look in the following folder and see how many thumbnails have been created:
/storage/emulated/0/Android/data/com.android.providers.media/albumthumbs/
In the Hi-Fi Cast app, you'll need to hit refresh from the toolbar menu to see any changes. It may take quite a while for your media to get re-scanned. As a very very rough guide, typical scanning speeds can be around 5 minutes per 1000 files.