This can happen for multiple reasons. Most likely, the author of the post did not adhere to standards and conventions. Both search services use pattern matching on the subject lines of posted messages, and this places a lot of responsibility on the author to stick to the conventions that are in place.
If you can't find an article, you can also try looking for it by browsing the newsgroup if you know which one it was posted in. Many posted messages are cross-posted to a few related groups.
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