WooCommerce: Variations are not showing in frontend

I have added an attribute Size in tab Attributes and added 4 values. Each value is separated by | sign. The options Visible on product page and Used for variations are also enabled.

On tab Variations all four values are added as a variation. All are enabled. But still on my front page I don’t see anything in drop down list. What am I doing wrong and how can I fix this. I am using the latest version (4.4.2) of WordPress and latest version (2.5.5) of WooCommerce.