I know there are a few threads already about this problem but mine seems a bit different.
I recently upgraded to WC2 and WP 3.6 together. Before the upgrade, things worked perfectly but now, the add to cart button doesn't show up on variable products. But here is the strange part.
If you refresh the product page once, using ctrl+f5 then the add to cart button comes up and works fine :s
these are the products that it doesnt show up on 1st time around:
it comes up fine on these, these are single products:
[Moderator Note: No bumping, thank you.]