After a lot of trial and error I figured out the problem was that every new created file or folder inside the /ftpdirectory were changing group ownership automatically to the user that added that files-folders.
I managed to set all newly created files-folders to be set down to ftp group ownership no matter who user, which of course belongs to the ftpgroup,created that files by these commands
chgrp -R groupname /ftpdirectory (to be sure everything is in the correct place)
and by using setgid
chmod g+s /ftpdirectory
After these changes until now and with many tests no issues so far.
I don't know if this action is the correct one but seems to work.
I have posted it for any future refference by any whom might find it usefull.
Thank again for your concern and effort to help.