Laravel on server fails find all the classes in app.css - css

Question

I'm designing a site using Laravel Mix on local. I have lots of scss lifes in resources/assets/sass and through app.scss I combine them it looks just the way it's supposed to be on local using npm run watch and php artisan serve
But when I set the site up on a server, all the css classes are missing, therefore design doesn't implement itself on the website. For instance I have a class future-concerts which styles the element it suggests. So I check for .future-concerts inside public\css\app.css there is no such class. What might be causing this? Even just copying app.css file to the server doesn't work. It just deletes most of the lines I wrote.
app.scss file
// Fonts
#import url('https://fonts.googleapis.com/css?family=Abhaya+Libre|Mitr|Raleway');
// Variables
#import "variables";
#import "custom";
#import "directives";
// Bootstrap
#import "bootstrap.min";
#import "bootstrap-theme.min";
// Fotorama
#import "fotorama";
// Animate.css
#import "animate";
// Font Awesome
#import "font-awesome.min";
webpack.mix.js file:
let mix = require('laravel-mix');
mix.js('resources/assets/js/app.js', 'public/js')
.sass('resources/assets/sass/app.scss', 'public/css');