the wild card doesn't work in gulp function

I have the following gulp script for conversting all .scss files from under app/scss/  to /app/css directory. I tried to use a single file name and it worked well.  Then, I switched to **/*.scss to try to catch all .scss files under the same directory. But it doesn't work. How could I trouble shoot for this error. I was following the gulp learning tutorial in this link: https://css-tricks.com/gulp-for-beginners/

Please help.

//for batch file converting from .scss to .css file
gulp.task('sass', function(){
  return gulp.src('app/scss/**/*.scss')
  .pipe(sass())  //Converts Sass documents to CSS documents with gulp-sass plug-in
  .pipe(gulp.dest('app/css'))
});