Hi, in my experience it shows up when attachment is either not downloaded yet for some reason or there's a problem with the attachment: it might be deleted but the link still points to local storage.

It does suppose to be clickable to force the download of the attachment but if you're running in "always download" mode and after checking it you have completed the sync the button should be replaced by an actual attachment.

If it's not, make sure the sync was complete and not interrupted by screen lock nor putting the app in the background. Sync on mobile cannot happen at those states. And with thousands of attachments it might take a long while. So it's best to setup the screen lock for as much as possible time and leave the app working in foreground.