grunt-contrib-less creates the app.css file during launch but not during live reload

Question

Basically, at the beginning all is ok but after modifications, CSS are not updated.

First, my package details:

"node" : "0.10.21" 
[...]
"grunt": "~0.4.1",
"grunt-contrib-uglify": "~0.2.7",
"grunt-contrib-jshint": "~0.7.1",
"grunt-contrib-concat": "~0.3.0",
"grunt-contrib-watch": "~0.5.3",
"grunt-contrib-less": "~0.8.1"

Next, my gruntfile.js

module.exports = function(grunt) {

grunt.initConfig({
    pkg: grunt.file.readJSON('package.json'),
    concat: {
        options: {
            separator: ''
        },
        dist: {
            src: [  'src/js/libs/jquery-2.0.3.min.js',
                    'src/js/libs/**/*.js',
                    'src/js/app/**/*.js'],
            dest: 'public/js/combo.js'
        }
    },
    uglify: {
        options: {
            banner: '/*! <%= pkg.name %> <%= grunt.template.today("dd-mm-yyyy") %> */\n'
        },
        dist: {
            files: {
                'public/js/combo.min.js': ['public/js/combo.js']
            }
        }
    },
    less: {
        compile: {
            options: {
                compress:true
            },
            files: {
                'public/css/application.css': ['src/css/application.less']
            }
        }
    },
    jshint: {
        files: ['gruntfile.js', 'src/js/**/*.js','src/css/**/*.less','public/index.html'],
        options: {
            globals: {
                jQuery: true,
                console: true,
                module: true,
                document: true
            }
        }
    },
    watch: {
        files: ['<%= jshint.files %>'],
        tasks: ['default']
    }
});

grunt.loadNpmTasks('grunt-contrib-concat');
grunt.loadNpmTasks('grunt-contrib-uglify');
grunt.loadNpmTasks('grunt-contrib-less');
grunt.loadNpmTasks('grunt-contrib-jshint');
grunt.loadNpmTasks('grunt-contrib-watch');

grunt.registerTask('default', ['concat','uglify','less','watch']);

};

Explanations

At first, all works perfectly, all the files are created:

Running "concat:dist" (concat) task
File "public/js/combo.js" created.

Running "uglify:dist" (uglify) task
File "public/js/combo.min.js" created.

Running "less:compile" (less) task
App runnning on http://localhost:3000
File public/css/application.css created.

Running "watch" task
Waiting...

But, if I make a modification in my *.less files:

OK
>> File "src\css\application.less" changed.

Running "concat:dist" (concat) task
Completed in 2.039s at Tue Nov 12 2013 11:44:53 GMT-0500 (Eastern Standard Time) - Waiting...

So, modifying is detected by watch but my CSS file is not updated and I just can't figure why. I tried several contrib-less implementations and it keeps the same way. I guess I'm doing something wrong but what ?

PS: concat and uglify are working very well.