The refresh_token is only provided on the first authorization from the user. Subsequent authorizations will not return the refresh_token again.
To get the refresh token again you need to "Revoke Access" next to your app. first and then The next OAuth2 request you make will return a refresh_token. This will prompt the user to authorize the application again and will always return a refresh_token
Only refresh tokens that are currently supported are the client credentials grant type, to read more click here. Use following to get the refresh_token.
POST https://api.omniture.com/token Parameters: grant_type:authorization_code code:someauthcodehere redirect_uri:http://someuri client_id:someclientid client_secret:someclientsecret