I already checked the file names, contents (whether they were blank files or not), properties, security options (read only, etc.), and ALL the files have the same properties. I tried changing the file names to all lower case but did not change anything, same with spaces...
In my folder for example, there are two .txt files : detail.txt and notes.txt. When it creates the txt subfolder, it only copies detail.txt to it. I don't understand.
Thanks for your answer!