I tried installing a flatpak with discover, and it silently failed. 
The progress bar showed some progress, then it disappeared from discover's in progress. Install button was still there, it then did nothing.


I then did it from the command line:


Looking for matches…
Required runtime for org.fritzing.Fritzing/x86_64/stable (runtime/org.kde.Platform/x86_64/5.12) found in remote flathub
Do you want to install it? [Y/n]:  

org.fritzing.Fritzing permissions:
    ipc      network              pulseaudio           wayland      x11
    dri      file access [1]      dbus access [2]

    [1] xdg-config/kdeglobals:ro, xdg-documents, xdg-run/dconf, ~/.config/dconf:ro
    [2] com.canonical.AppMenu.Registrar, com.canonical.AppMenu.Registrar.*


        ID                                      Arch     Branch  Op  Remote   Download
 1. [✗] org.kde.Platform                        x86_64   5.12    i   flathub    706.6 kB / 425.9 MB
 2. [ ] org.freedesktop.Platform.VAAPI.Intel    x86_64   18.08   u   flathub    < 1.8 MB
 3. [ ] org.freedesktop.Platform.html5-codecs   x86_64   18.08   u   flathub    < 4.9 MB
 4. [ ] org.kde.KStyle.Adwaita                  x86_64   5.12    i   flathub    < 4.7 MB
 5. [ ] org.kde.Platform.Locale                 x86_64   5.12    i   flathub  < 334.0 MB (partial)
 6. [ ] org.fritzing.Fritzing                   x86_64   stable  i   flathub   < 31.3 MB

Error: Not enough disk space to complete this operation
error: Failed to install org.kde.Platform: While pulling runtime/org.kde.Platform/x86_64/5.12 from remote flathub: Writing content object: min-free-space-size 500MB would be exceeded, at least 32.8 kB requested


Which shows the problem discover must have hit