We (our company) has been using one CMS for several websites that we have created. We have encountered this particular problem several times now and we cant think of a fix for it.
Here's the description of the problem: In this cms there is possible to create picture galleries. When you create folder for gallery via CMS then folder is beeing created by www user and its permissions are set to 0666. When user uploads pictures to this gallery using its editor view, then the pictures are uploaded to previously created folder and cut into necessary sizes. .thumbnails and other folders are created to hold pictures of different sizes.
If you create folder for gallery via ftp, then the owner of the gallery is ftp user. If you manually set permissions to 0666 and upload the files with correct permissions, everything seems to be fine. Now in the public view, if you choose this previously created folder as your gallery folder, the cms fails to create .thumbnail subdirectory (and other required subdirectories also). The only difference between the folders is the owner of the folders.
Right now i am trying to get my hands on this hosting accounts php log. I guess i will have more information then.