not allowed to modify permissions from 777 to 755

hi,
Something very weird happened. I have a site using Graphene theme and I started modifying it from a different PC as it has been before. First problem it was that I couldn’t upload pictures and the error given was about permissions. I checked and it was set to 755 for uploads file. I have tried several times with no results. So I changed the permissions to 777 with the intent to put it back to 755 after uploading the images. What happened was that a new file has been created 2013 with 01 subfolder (as the year and month as wordpress organizes images in such folders) with permissions 777. I was able to upload the images but when finished it would not let me modify it to 755. I have tried with web server’s FTP as well as with FileZilla but no results. However I could change it for the uploads file but not the subfiles.
What has happened? Could it be that because the permissions were set to 777 when it has been created I am not allowed to modify?
Any ideea? Thank you