Before assets can be viewed, they need to be downloaded.
Depending on the size of the asset and your internet connection, this can take up to a few minutes.
Downloads will be automatic when connected via Wi-Fi, if you have enabled this in the Account screen, otherwise you need to tap the tile's download icon to download it.
NOTE: Each asset has its file size displayed, which provides an indication of the necessary download time.
iOS/Android:
Desktop: