Stockable variant can't sum the quantities...

[Beni]

Hi,
A have a stockable variant in my store. If I click add to cart and again, this product show multiple in cart...! Not sum the quantities? Why?
Az original cart variant is work well. Please help me! See image: http://oi44.tinypic.com/2iqhgk7.jpg
Sorry for my English.
Beni

[Beni]

Anyone seen this before?
If need, I can send access the store! Please help me.
Beni

[Beni]

I tried with Beez5 default template, but all is same...
See my demo site: http://www.demovm.butor-vasalat.hu/webshop/handtools/nice-saw-detail

Although, the other products add to cart and sum in the cart well. And the cart variant custom field is work well, eg:
http://www.demovm.butor-vasalat.hu/webshop/powertools/outdoortools/chain-saw-detail

Anyone can help?
Thanks