Grunt Concat Does Not Unify Files

Question

I want to unify my css files into one, but Grunt doesn’t unify. It works perfectly for javascript.

Follows:

concat: {
    options: {
        separator: '\n\n',
    },
    css: {
        src: [
            'dist/app/css/*.css'                
            , 'dist/app/lib/boostrap/dist/css/bootstrap.css'
            , 'dist/app/lib/boostrap/dist/css/bootstrap-theme.css'
        ],
        dest: 'dist/app/css/all.css'
    },
    js: {
        src: [
           /* Uma grande lista de arquivos omitidos */
        ],
        dest: 'dist/app/js/all.js'
    }
},
/* código restante omitido, mas também funciona corretamente */

No errors are displayed on the console. Even running the command grunt or grunt concat:css.