Darwin,

Have you configured swish-e to index that specific meta tag?

Read swish documentaion below:

#################################################
The new "PropertyNames" tag in the configuration file lists all META tag
names that are to be retrievable at search time.
Note: the index must be regenerated for this to have any effect. 

This example defines three document properties: 

PropertyNames description author datamodified

This is a META tag example that sets the value of the "author" property
to "Jefferson" and the "description" as shown:

<meta name="author" content="Jefferson"> <meta name="description"
 content="Blueprints"> <meta name="datamodified" content="Jul-4-1776"> 

A new command-line option, "-p", is used to specify which document
properties are to be included in the search results
 output. Follow the "-p" option with property (meta) names. 

This example will return two document properties: 

swish-e -w Independence -p description author
##################################################