Not all selected iPhoto pictures upload on Flickr.

I've just signed up with Flickr account, since it allows direct download of pictures from iPhoto '11 app. However, I've noticed a peculiar bug: Say I selected 40 pictures to be uploaded to Flickr; when the task finishes there are only 37 pictures showing up in the Flickr! It happened with 3 different albums and missing pictures are totally random. Any suggestions as to what is happening?

Thank you.