Make sure In Stock (products_status = 1) is turned on ...
Make sure Stock (products_quantity = 0) is set to 0 ...
Make sure that you have configured in the Stock setting is set to 1:
Quote:
Products status in Catalog when out of stock should be set to
Show Products when out of stock
0= set product status to OFF
1= leave product status ON
Now items that sell to <= 0 will be set to show the Sold Out button automatically on Add to Cart and Buy Now buttons/input boxes ...
NOTE: Stock tracking must be turned on ...