Hey there,
Two things I recognized:
First: I tried to add source links to some of my upload, as I forgot to add them during the upload. It seems that it is not possible to add source links with special characters in it, like Russian characters or French ones. It always ends in an error message, which states that the URL is wrong. It is no problem to add those links, if you do it during the upload process, but if you try to add it later, it just does not work. However, you can add links without special characters after the upload.
Second: As the dupe detection did not work on some of my uploads, a few images have been merged later. What I recognized was that you do not merge the source with it. I know, that quite a lot (not all) uploaders on here don't care for the source, even they download their images definitely from those sources, and that you as the site owners don't care much either, but I personally do care. For me it is a question of respect. In the end, most of us do steal those images and violate copyright in many ways. I can live with that, as I am doing it too. However, giving credit is the least one can do! What I am trying to say here - Would it be possible to keep the source link, when you merge too images and one of them is having such a link?
Thanks for your help! :)