Dropbox - cannot open all of my photos on android phone, offline?

[Forest 1]

I have dropbox on my android phone. When I am online I can see all of my photos no problem, however when I am offline only a few of them are viewable.

Most of them say 'Thumbnail failed to load', which is weird because SOME of them open fine, even those with virtually the same file name etc.

Any idea why?

[Tycho]

Dropbox only shows a "ledger"/index of the photos you have stored in your Dropbox.
When you open a photo when online, it downloads that photo to your phone from the Dropbox servers so that photo is then saved on your phone.
But when you're offline, your phone can't reach the Dropbox servers and thus only shows the photo's you have opened in the past (and are saved on your phone because of that reason)

[Jan]

Because your files are not marked as favorites.
Once you mark a file as a favorite, you'll be able to access it on and offline.