jQuery json loop with ajax call for php function

I'm new to Javascript. The code for a jQuery-ajax-php-cooperation does strange things. It works - sometimes.

This is what I want to do:

• make some settings in a form (works)
• read JSON-file after pressing submit-button (works)
• loop the JSON-items, extract the values of each item and form a parameter-string (problem is here: loop is not (always) executed in the button-pressed-function)
• send parameter-string built from each items values to PHP-file (works if loop is entered)
• receive the response from the PHP-file (html-tag) (works)
• update a form element with the response value (not yet implemented, actually show an alert with the PHP-response for debugging purposes)

The JSON file is valid (tested). The HTML-Page is valid HTML5 (tested). The PHP-script works and returns a valid HTML-Image-Tag (tested).

When I press the button, no responses from the PHP-file are displayed in the alert I implemented for debugging purposes. But this works:

• reload page
• open Firebug
• set breakpoint at loop-begin
• skip one statement forward
• reload page
• the loop is entered, all works fine

I can close Firebug and the loop is performed properly.

The javascript code

<script type="text/javascript">
$(document).ready(function () {
   //click event of submit button
   $('#generator').click(function(){

   // GET variables
   var datafile = "my.json";
   var logo = false;

   // if checkbox is checked
   if( $('#logo').attr('checked')){
      logo = true;
   };

   // read data from json file
   $.getJSON(datafile,function(data){

   //iterate through the data-sets
   for(var i=0; i < data.length; i++) {

    // get the response from server for each data-set
    $.ajax({
       type: 'POST',
       async: false,
       cache:false,
       url: 'myfile.php',
       data: 'param1='+data[i].field1+'&param2='+data[i].field2+'&logo='+logo,
             success: function(response){

              //$('#imgdisplay').html(response);
              // alert only for debugging purposes
              alert(response);}

          }); // end of ajax-call
       };  // end of for-loop
   }  ); // end of getJSON
 }); // end of button.click
 }); // end of document.ready
</script>

(sorry for bad intented formatting)

I use a for-loop, could not get an each()-loop to work properly.

And this is the form:

<form name="settings">
   <label>Source file</label>
   <select id="datasource" name="datasource">
      <option value="extract">Extract</option> 
      <option value="filter">Filter</option>
   </select><br />
   <label>Logo</label>
   <input type="checkbox" id="logo" name="logo" value="ON"/><br />
   <p>&nbsp;</p>
   <input type="submit" value="Start Generator" id="generator" name="generator" />
</form>
<div id="imgdisplay" class="imgdisplay"></div>

What causes this strange behavior? How can I solve it?