Problem with Prototype request callbacks

Hi,
I have a rails app and a mobile app. My web app is using javascript so I make jax request to get my data.
My problem is that I submit a form but I can't make work the callbacks even if the object is returned properly.

    $('new_user').request(HOST +'users',{
      onSuccess: function(transport){$('all').insert(transport.responseText);  },
      onComplete: function(transport){ $('all').insert(transport.responseText); }
      });

The errors aren't working, I don't know why...

Thanks,
Greg