Эликсир не минифицируется и не комбинируется

привет, я пытаюсь минимизировать и объединить свои файлы в один файл js и css, но он не работает, когда я запускаю gulp --production my gulpfile.js

var elixir = require('laravel-elixir');

elixir(function(mix) {
     /**
     * Copy needed files from /node directories
     * to /public directory.
     */
    mix.copy(
       'node_modules/font-awesome/fonts',
       'public/build/fonts/font-awesome'
     )
    mix.copy(
       'node_modules/bootstrap-sass/assets/fonts/bootstrap',
       'public/build/fonts/bootstrap'
     )
    mix.copy(
       'node_modules/bootstrap-sass/assets/javascripts/bootstrap.min.js',
       'public/js/vendor/bootstrap'
     )

    mix.styles(['frontend.css', 'style.css', 'responsive.css'], 'public/build/css/frontend.css', 'public/css')
    .styles(['backend.css'], 'public/build/css/backend.css', 'public/css');
    mix.scripts(['frontend.js'], 'public/build/js/frontend.js' , 'public/js')
    .scripts(['backend.js'], 'public/build/js/backend.js', 'public/js');

    mix.version(["public/css/frontend.css", "public/js/frontend.js", "public/css/backend.css", "public/js/backend.js"]);
});

и мой консольный вывод

➜  public_html gulp --production
[19:29:22] Using gulpfile /home/admin/web/***.com/public_html/gulpfile.js
[19:29:22] Starting 'default'...
[19:29:22] Starting 'copy'...

Fetching Copy Source Files...
   - node_modules/font-awesome/fonts/**/*


Saving To...
   - public/build/fonts/font-awesome

[19:29:22] Finished 'default' after 49 ms
[19:29:22] Finished 'copy' after 80 ms
[19:29:22] Starting 'copy'...

Fetching Copy Source Files...
   - node_modules/bootstrap-sass/assets/fonts/bootstrap/**/*


Saving To...
   - public/build/fonts/bootstrap

[19:29:22] Finished 'copy' after 8.26 ms
[19:29:22] Starting 'copy'...

Fetching Copy Source Files...
   - node_modules/bootstrap-sass/assets/javascripts/bootstrap.min.js


Saving To...
   - public/js/vendor/bootstrap

[19:29:22] Finished 'copy' after 3.33 ms
[19:29:22] Starting 'styles'...

Fetching Styles Source Files...
   - public/css/frontend.css
   - public/css/style.css
   - public/css/responsive.css


Saving To...
   - public/build/css/frontend.css

[19:29:29] gulp-notify: [Laravel Elixir] Stylesheets Merged!
[19:29:29] Finished 'styles' after 7.12 s
[19:29:29] Starting 'styles'...

Fetching Styles Source Files...
   - public/css/backend.css


Saving To...
   - public/build/css/backend.css

[19:29:40] gulp-notify: [Laravel Elixir] Stylesheets Merged!
[19:29:40] Finished 'styles' after 11 s
[19:29:40] Starting 'scripts'...

Fetching Scripts Source Files...
   - public/js/frontend.js


Saving To...
   - public/build/js/frontend.js

[19:29:40] gulp-notify: [Laravel Elixir] Scripts Merged!
[19:29:40] Finished 'scripts' after 866 ms
[19:29:40] Starting 'scripts'...

Fetching Scripts Source Files...
   - public/js/backend.js


Saving To...
   - public/build/js/backend.js

[19:29:41] gulp-notify: [Laravel Elixir] Scripts Merged!
[19:29:41] Finished 'scripts' after 610 ms
[19:29:41] Starting 'version'...

Fetching Version Source Files...
   - public/css/frontend.css
   - public/js/frontend.js
   - public/css/backend.css
   - public/js/backend.js


Saving To...
   - public/build

[19:29:41] Finished 'version' after 52 ms

в основном он должен минимизировать и объединять файлы css и js в один файл css и js.

но style.css и responsive.css не объединены в файле frontend.css

плиз помогите что я делаю не так?