There are a couple reasons why an item will show as "Out of Stock" on the front end of a website.

(1) The Item Stock Status is set to "Out of Stock"

Even if a product has Quantity the Stock Availability (In Stock/Out of Stock) takes precedence

(2) If it is a Configurable product it could also be a result of there being NO Associated Simple Products.

As can be seen below there are "No Records Found". A possible reason for this is that this Configurable Product should only have "Color" as a valid Super product attribute instead of both "Color" and "Size".

In order to fix this problem the Configurable Product MUST be deleted in Magento and recreated with the correct Super product attribute.