can not select last child product on stock

Hello,
I'm strugling with some parent and child products.
What i have experienced is that my last product on stock cannot be ordered because it says no stock anymore.

Here my example:
www.dodowear.nl/index.php/meisjes-kledin.../category_pathway-55

In fact what the status: all other sizes have been sold. There only only one at stock in a particular size. But I can't select this size because it thinks there is no stock anymore. But there still one.

What should be changed?

Thank you in advance

Hi,

I don't have the problem.
If I change the size on that product page, it says that there is one product left and I get the add to cart button. So it seems to be working fine.