I can successfully upload files but they never show up. I can view them with an ftp client, and silverstripe sees the names of all of them. There is just never a thumbnail and if I link to one it appears as a broken link. Any ideas. I'm not running any mods and I'm pretty sure I have the folder permissions right.
What version are you using? I recently upgraded a dev site to 2.2.2rc2, and am having the same problems, however, I haven't had any problems with sites running on previous versions.
I'm only running 2.2.1
I cant be able give a deep answer but got 2 things which I know to cause this.
1. If you uploaded the files via FTP (not SS upload module) you need to go to the Files&Images section, browse the related folder and let SS to see files in this browser for one time. Than it starts to show them in other menus also
2. And other is my general experience on other scripts that I used or tested (not with SS but)
If you use folders or filenames with non english weird characters or a space in names, most of scripts see them but couldnt make a thumbnail. Maybe about image rendering modules in the server but it happens like that.