Is it possible that google news does not index every article a source of theirs publishes?
Amazon doesn't get all of its pages indexed in Google, and it's quite likely that a news source doesn't get all of its articles indexed... but I can't say that with any certainty.
You might try running some tests. Identify a site comparable in authority and size to the site you're evaluating, and then run comparable searches. I'd search for headlines or text strings in articles of comparable age, with and without quotes.
Also, try a site:domain type search (ie, use the search operator), in this format... site:example.com "quoted query string here"
Note that there should be no space between the site:
operator and the domain.
It's possible for topical web content, once it's off the front page and has disappeared from current news, not to show up in a search of the entire web... unless it received some external links or the site itself is well optimized. You may not have a way of judging that.
Both the quoted search (for a unique
text string) and the site:
operator search help narrow that down considerably... and would be a better indication of what's in the index.
I'd search for old articles in the regular google.com, not in news.google.com.