JQuery :: Load Function Works Once Only

I've got this code to work the first time that the user changes the select option:
$('select.third, select.fourth').change(function(event){
var id = $(this).siblings('.hidden').text(),
token = $('#token').val();
param = $(this).hasClass('third') ? 'isSold' : 'isActive';
value = $(this).val();
$('div#allSheepDisplay').contents().remove().end().load(
'db-interaction/animals.php',{
action: 'update',
id: id,
token: token,
param: param,
value: value
}, function() {
$('select.third').filter(function(){return $(this).val() == '1'; }).css({backgroundColor: 'yellow'});
$('select.fourth').filter(function(){return $(this).val() == '0'; }).css({backgroundColor: 'red'});
}); });
Everything works as I expect - the POST request is made correctly, the database is updated and the display updates correctly. However, if I repeat the same action, selecting a different value, nothing happens until I refresh the page, at which time I can then make the change.