Bug: Product variation does not work

I have three t-shirts size (M, L, X-L)and different quantities (4, 8, 12)

Whenever I try to manage the stock, woocommerce can only register the last value. I.e If I put M =4, L = 8, 12, X-L, the system will register the last value (12) for all of them.

Need help…Thank you

http://ww.wp.xz.cn/extend/plugins/woocommerce/