In the backend of Magento, go to Catalog > Attributes > Manage Attributes for an overview of your attributes. Clicking on an attribute will give you more details about its configuration:
In order to show an attribute in the layered navigation it is mandatory to configure the ‘Catalog Input Type for Store Owner’ of the attribute as dropdown, multiple select or Yes/No. Also you can chose to make the attribute visible on the product detail page by selecting ‘yes’ in the dropdown next to “Visible on Product View Page on Front-end”.
As a consequence the frontend of Magento will display the value of the attribute picked by the store owner when creating the product. However, when no value is chosen by the store owner concerning the above mentioned attribute types, the frontend will display the word ‘NO’:
In order to prevent this one can do 2 things:
1. Oblige the store owner to enter a value to the attributes when creating the product in Magento. This can be done on the product attribute details: setting “Value required” to yes
2. You can change the php-code of Magento. Use the local folder to do this!