AJAX Event Not Working

AJAX Event Not Working

I am new to jQuery and I am trying to do an AJAX call. The only call I could get to work was getJSON (which is fine because my responses are always JSON). Firebug lets me know that the request goes out and the response that is returned is what expected, but the success function is never called. Based on the code below, I should get an alert with the response, but I don't. If the problem is a JSON parsing error, how do I find that out? If something else is the problem, what could it be?
  1. $.getJSON (
                    prefix + "getJSON.php",
                    {
                        parameter: ''
                    },
                    function (json) {alert (json);});