WooCommerce "Add to Cart" Button removed after applying variation product

Hi
I use WordPress 3.6.1
WooCommerce plugin 2.0.17

my “add to cart” in woocommerce button was removed in some product pages. In simple product page, “add to cart” button is exist. In variable/variation product page (size, color, etc) “add to cart” button is missing and replaced

in simple product :
http://i1349.photobucket.com/albums/p756/Afif_Fauzi_Auliaurrahman/bener_zps410d26a7.png

in variation product (button is missing)
http://i1349.photobucket.com/albums/p756/Afif_Fauzi_Auliaurrahman/Errorvariation_zps8c839201.png