At least for me. I upload large batch of pictures and two random appear in my queue. When I connect to FTP server my directory there is empty. It must be some very smart processing in place :-)

FTP did stop working about 2 weeks ago but it was only down for a couple of days but since then I find if I upload SVG files and jpeg (not for the same file) only one type will appear i.e. all jpeg or all svg files.

Answer from customer support:
"The engineers suggestion is to use: Fetch or Filezilla (free software)in order to upload your files.
You can download filezilla from: http://filezilla-project.org/"

I am using same software (Sitecopy) for every site and only Fotolia got this peculiar behavior. Asked them more technical questions so maybe I have to tweak my configuration (PASV, CWD)

"Results are reported in root's mail". Your upload attempts should be logged, anything there?

No, I mean on your local system, the docs for Sitecopy indicate that transfers are logged.

You can send root mails to your email by a technique such as : http://www.ubuntugeek.com/howto-forward-roots-mail-to-your-inbox.html

What about Mirror.

"You can use mirror instead of sitecopy - the basic functionality is duplicated. But, sitecopy does not go to the FTP server and see what's there every time - this is the basic difference between sitecopy and mirror. This saves bandwidth, and startup time, especially so for slow servers holding large sites. As well, sitecopy has some features missing in mirror (though the reverse is also true), such as remote file move and interactive confirmations." - so, worth a try, if only perhaps on the one site.

http://www.bio-mirror.net/biomirror/software/mirror-info.txt or perhaps http://rumkin.com/software/mirror/mirror.txt

Looks like default setup does not include code for multiple remote sites, but you could set up a separate cronjob just for the Fotolia one + Mirror and see if that resolves it.

If you're a Perl whiz, (and you find Mirror more reliable)  you could easily modify the standard Mirror code to include multiple remote sites, or just be a lazy and create one cronjob for each site you're uploading to. That might even work out for the best, if the remote physical location of the remote servers is distributed across different timezones. I'd aim for 4 AM localtime for each server.

Well looks like theres room in the market for a multi-OS, multi-agency scheduled uploader desktop app then?  I'm sure the majority of photographers didn't know what . we were just talking about, seems like a 'drop your photos in this folder and they get FTP'ed' utility would be a winner..

Mystery solved. I guess they silently delete files that do not meet their size requirements. I checked and I was uploading 2MP files and they require 4MP now.  I only hope customer service would have any record of this and I would not lost time posting files to void