jQuery：JSON停止工作

The code below just stopped working, although I haven't even touched this part of the file since last time it worked.

$.post(PATH_PRODUCTS_AJAX_URL, att, function(data) {
  if(data.proper)
  {
    current_step = data.step;
    $('#product_container').append('<div class="product_content">' + data.content + '</div>');
    $('#product_container .product_content:last').animate({'left' : '-=460px'}, 200, 'swing');
  }
  else
  {
    alert('Not proper.');
  }
}, 'json');

My debug process is as follows:

I've checked the URL and the input data - they're both fine. I use the data.proper variable to ensure that the data is alright; if the data isn't processed the right way, I should get the "Not proper" alert box - but now I don't. I've also tried with a random alert box outside the if statement, but nothing happens at all.

But if I remove the JSON parameter, I get the "Not proper" box. I could also see that the output data is all fine by alerting the data variable.

I've checked the involves files, and they're all UTF-8 encoded.

I have really no idea what I should do next. Any help appreciated.

Edit: (example data)

products.php:

$return['proper'] = true;
$return['step'] = 1;
$return['content'] = 'Hello';

$json = json_encode($return);
echo $json;

Response (note that this is only returned when the JSON parameter is excluded):

{"proper":true,"step":1,"content":"Hello"}

Could you try changing from the shorthand .post() to an .ajax() call to give you access to show the error:

$.ajax({
    type: "POST",
    url: PATH_PRODUCTS_AJAX_URL,
    data: att,            
    dataType: "json",
    success: function(data) {
        if (data.proper) {
            current_step = data.step;
            $('#product_container').append('<div class="product_content">' + data.content + '</div>');
            $('#product_container .product_content:last').animate({'left': '-=460px'}, 200, 'swing');
        }
        else {
            alert('Not proper.');
        }
    },
    error: function(XMLHttpRequest, textStatus, errorThrown) {
        alert("Response text: "+XMLHttpRequest.responseText);
        alert("Status returned: "+textStatus);
        alert("Error thrown: "+errorThrown);
    }
});