I have a product item on my site that is not computing tax, how can I correct this?

Besides setting the tax methods and rates on the site under the sites Settings > Tax tab, you can also override this setup under the sites Catalog > Products > 'product item' > General tab > Tax area > Override Tax setting.
Check to see if this setting is set to 0.00. This would override the sites tax calculation and calculate 0% tax for this product item. Remove the 0 value for this setting and Save the product item. Tax for this item will then be calculated from the sites Settings > Tax tab setup.