Auto-Upload ignoring one file

How can I find out why the android app’s auto upload is ignoring a file?

It uploaded the files before the one and after that one. It shows it in the 8 picture preview when opening the auto upload settings.
It is not big, just a picture.
It is not shown as failed upload in the upload log.

This really makes me nervous since if it is not shown in the log as failed, it might stay unnoticed until it is removed from the device and thus gone.