If you plan on being offline and use the app, it is always a good idea to start by manually checking for updates in settings before hand. Even if you have the toggle set to automatically check for updates, we recommend manually pressing the button to ensure every available item is downloaded.
If you cannot view an item while you are off-line, chances are that piece of media is not available off-line.
There will be a small orange WiFi icon in the corner of the thumbnail that tells you if it is not available off-line
The media file will be able to be downloaded once you are connected to a network again.