Plumber show error only 1 file - gulp-sass #59

Hi, I need a little help, when we change something especially in the main.scss file there will be an error, when we change something in another file there is no error at all, where can the error be?

 gulp.task( "css", function() {
    gulp.src( "assets/scss/main.scss" )
    .pipe( plumber() )
    .pipe( sass.sync({
        outputStyle: 'compressed'
    })
    )
    .pipe( autoprefixer({
        browsers: ["last 8 version", "IE 9"]
    }))
    .pipe(concat('main.min.css'))
    .pipe( gulp.dest( "dist/css/" ))
    .pipe( browserSync.stream() );
});
gulp.task( "watch", function() {
    gulp.watch( "assets/scss/**/*.scss", ["css"] );
});

main.scss

// Global
@import "global";
//Header
@import "header/header";
@import "header/mobile-menu";
//Front-page
@import "front-page/front";
// Footer
@import "footer";