Grunt: grunt-hub isn't watching all projects

I've installed grunt-hub into my workspace, which looks like this

hub/
node_modules/
    grunt/
    grunt-hub/
Gruntfile.js
package.json

In Gruntfile.js I have written this,

module.exports = function (grunt) {
    'use strict';
    grunt.initConfig({
        pkg : grunt.file.readJSON( 'package.json' ),
        hub: {
            src: [
                'hub/*/Gruntfile.js'
            ],
            watch: {
                src: '<%= hub.src %>',
                tasks: ['watch']
            }
        }
    });
    grunt.loadNpmTasks('grunt-hub');
    grunt.registerTask('default', []);
}

I have four files within the hub directory, which have their own Gruntfiles.

hub/
    project1/
        ...
        Gruntfile.js
        ...
    project2/
        ...
        Gruntfile.js
        ...
    project3/
        ...
        Gruntfile.js
        ...
    project4/
        ...
        Gruntfile.js
        ...

When I run...

grunt hub

...it works perfectly fine; it watches all the changes I make and runs how I've commanded them to run.

The only problem is in the command prompt I am told...

>> C:\Grunt\hub\project1\Gruntfile.js:

Running "watch" task
Waiting...

>> C:\Grunt\hub\project2\Gruntfile.js:

Running "watch" task
Waiting...

>> C:\Grunt\hub\project3\Gruntfile.js:

Running "watch" task
Waiting...

...but am not told that project4 is being watched. When I make changes to files relating to project4, nothing happens, whereas it does for everything else.

What can I do to make it watch project4 as well?