add_shortcode() not working inside of function

Just use as per the following code sample in your plugin.,

function wp_shortcode( $atts ) {
    extract( shortcode_atts( array(
        'foo' => 'something',
        'bar' => 'something else',
    ), $atts ) );


    //return "foo = {$foo}";
    return your_function();
}
add_shortcode( 'your-shortcode', 'wp_shortcode' );

I think this may help for you to get your needs.