Form submits with a varied success PHP+IFRAME+AJAX

Question

I have a 3 steps checkout.

1. STEP: User upload a photo and crop it.

2. STEP: User add his shipping information.

3. STEP: User pay with PayPal.

Now, i want to save an information about the order in two steps.

I save the image information + upload image using iframe:

 <form id="upload_form" enctype="multipart/form-data" method="post" action="<?php echo get_template_directory_uri(); ?>/upload.php" onsubmit="return checkForm()" target="image_upload_frame">

 <iframe name="image_upload_frame" height="1" width="1" style="visibility: hidden"></iframe>

My upload.php upload the image to /cache/ directory and put data into database about link, framesize, and price, then it echo row_id of the last row.

But it works sometimes and sometimes not! I mean I some time it do save information in DB and in some times no.

Any suggestions?

Here is another peace of code, where i call to .submit() function

$(".steps a").click(function(){
                    $(".steps a").removeClass("selected");
                    $(this).addClass("selected");
                });
                $("#st1,#0stepnext").click(function(){
                    $('#upload_form').submit(function(){
                        $("#pagestep1").show();
                        $(".scrolldown").show();
                        $("#pagestep2").hide();
                        $("#pagestep3").hide();
                    });

                });
                $("#st2, #1stepnext").click(function(){
                    $("#pagestep2").show();
                    $("#pagestep1").hide();
                    $("#pagestep3").hide();
                    $(".scrolldown").hide();
                });
                $("#st3, #2stepnext").click(function(){
                    $("#pagestep3").show();
                    $("#pagestep1").hide();
                    $("#pagestep2").hide();
                    $(".scrolldown").hide();
                });

Here is a part of my php:

$sImage = uploadImageFile();
global $wpdb;

$price = (int)$_POST['price'];
$frame = $_POST['filedim'];
$link = $sImage;
$id = $wpdb->insert_id;

$wpdb->insert('orders',
    array(
        'price'=>$price,
        'frame'=>$frame,
        'link'=>$link

    ),
    array(
        '%d',
        '%s',
        '%s'

    )
);
echo $wpdb->insert_id;