it might be the the 'config.php' file doesn't have write access to it.
I forget what is the 'best' secure permission, but if you do:
chmod 777 config.php (in the console)
you should be ready to go.
basically what you are doing is giving everyone read/write/execute access to that file.
if you have console access, do:
man chmod (gives info about: file permissions)
man chown (gives info about: ownership of file)
hth.