Productimage doesn't display sometimes

Hello,

I’ve just added 4 products to my webshop and 2 of them display as I suspect and 2 don’t. They don’t display the image.

URLs:
http://tinyurl.com/gptmyk4 (wrong version)
http://tinyurl.com/jqtewm7 (correct version)

It must be something in the productconfiguration that makes the difference, because of that I see this:

<div class="images">
    <div id="ss_wrap">
        <div class="flexslider">
            <ul class="slides">
    </ul>

AND

<div class="images">
    <div id="ss_wrap">
        <div class="flexslider">
            <ul class="slides">
        <li><div class="single-product-image">
      		<a href="http://www.by-nique.nl/wp-content/uploads/2016/01/Best-basic-large-blue1.jpg" rel="shadowbox[]" ><img width="440" height="440" src="http://www.by-nique.nl/wp-content/uploads/2016/01/Best-basic-large-blue1.jpg" class="attachment-650x440 wp-post-image" alt="Best basic large blue1" /></a>
        </div></li>
       	    </ul>

So you see that the difference is made in the code.

Both products have 1 product image and they’re both variable products that belong to 1 category. So I don’t see any difference at all that can trigger this problem.

Can anybody tell me where to look for the answer?

https://wordpress.org/plugins/woocommerce/