php - wp_ajax_ 400 Bad Request

Trying to run a ajax request back to a function and keep getting a 400 error. this is the code below

   wp_register_script('frontend_js', plugins_url('/js/frontend.js', __FILE__), array(), false, true);

   wp_localize_script('frontend_js', 'frontend_ajax', array(
       'ajax_url' => admin_url( 'admin-ajax.php' ),
       'nonce' => wp_create_nonce('stuff here')    
    ));
     wp_enqueue_script('frontend_js');

I have then got a function and using add_action to register it

 add_action('wp_ajax_processingHook','processingHook');
 add_action( 'wp_ajax_nopriv_processingHook','processingHook');

 function processingHook() {
     echo "111";
 }

And then in the jQuery

jQuery.ajax({
   type: "POST",
   dataType: "json",
   url: frontend_ajax.ajax_url,
   data: {
        action: 'processingHook',
        newForm: formstuff,
        security_nonce: nonce

    },
    success: function (msg) {
        alert(msg);

        // Do something when done
    },
    error: function (xhr, status, error) {
        var err = JSON.parse(xhr.responseText);
        console.log('err = ' + err);
        console.log('status = ' + status);
        console.log('error = ' + error);

    }
});

And I get a 400 error.

I have a very similar ajax working in the admin pages successfully. Using console.log I get the correct ajax url and when I browse to the admin-ajax.php url I get a 0 on the page.

Am I missing something running ajax in the frontend of of a wordpress site ? is there a permission or something that needs to be set to send data to via ajax ? Or is running 2 separate ajax calls like this, backend front end, problematic ?

Cheers

EDIT: - Some clarification there are two files admin and front end.

Admin file contains an ajax function that works. Frontend file contains the above function, processHook()

As I said below in comments I moved the processHook() function and add_action(wp_ajax into the admin file and IT WORKED. I left the wp_localize call in the frontend file.

Can anyone explain this ? Does it have anything to do with the way and timing of the files being loaded ? Is there meant to be a priority to the add_action(wp_ajax_ calls ?