Googlebot (Google's Spider) doesn't follow the "http/1.1 302 Found" status code (resource temporarily moved). Instead you should use a "http/1.1 301 Moved Permanently" header to make Google follow the redirect.
To make the long story short: If you're using PHP to do the redirect (and many are using PHP these days) you should add the status code header manually because PHP sends a 302 Found status code by default.
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