When you use the command line to do this, it sets the variable for your current shell. If you switch to another terminal and echo $PATH, you'll see it hasn't been changed there.
You need to add the changes to the right config file for your shell. For example, if you use bash, look in your home dir for .bash_profile or .bashrc - one of those files will have some commands that are always executed in a new shell. Add your updated PATH line to that file, then log out and back in again to test.