I can only find one Tweet with that term in the past seven days (the search widget only works with a 7 day index). It’s a retweet of content from Sept 5. The account that posted the retweet has only that single Tweet in its timeline and very few followers, so my guess is that it is not part of the partial search index used by the widget.
You may want to consider curating a Collection timeline to ensure that the Tweets you want displayed on your site appear more predictably.