The 755 permission gives the group/others the read and execute permissions in the directory.
This means, that non-group members cannot create new directories.
Make sure that the owner of the directory is the user that the server is running as.
To figure out which user that is, you can use:
$ echo $(ps axho user,comm|grep -E "httpd|apache"|uniq|grep -v "root"|awk 'END {if ($1) print $1}')
And if does not provide the desired result, simply explore the output of:
$ ps aux | grep -E "httpd|apache" | grep -v -E "root|grep"
You can find which group it belongs to by using:
$ groups [userName]
Next, change the owner of the joomla folder. I am using www-data
as an example:
# chown -R www-data:www-data path/to/your/joomla/root/dir
PS,
lines preceded by $
can be executed by a normal user, lines preceded by #
require root privilege - you can use sudo
or your favorite method.