Hi,
Well, the problem is that there is not enough space to display the -, the +, the quantity input field and the add to cart button in that area with the size of each elements:
take.ms/lIWxf
As you can see, the add to cart button goes below and stays hidden because of that.
In your place, I would reduce the width of the quantity input field and remove the margin/padding on the add to cart button div. That would result in this:
take.ms/JylkR
Then it's just about arranging the look of the different elements the way you want.