Menu error

I have a problem at menu.
Screenshot:
http://i1241.photobucket.com/albums/gg512/tuanbao92/Error_zpsr5lk6gdw.png
Sản phẩm is parent page and Thiết kế is sub-item

I want to hover Sản Phẩm, Thiết kế will show.
Sorry, I’m bad English