It's likely just a case where it's not being returned from Twitter Search. There can be hundreds, or even thousands of hits for a hashtag search and Twitter does not return all of them. We only display what we receive from their search. So it doesn't mean the tweet isn't there, just that it's not being returned in search.
Here is Twitter's documentation on searching the API:
Question: I Don't appear in Search and my hashtags and mentions don't work. Why are the Tweets I'm looking for not in Twitter Search, the Search API, or Search widgets?
Answer: Twitter's search is optimized to serve relevant tweets to end-users in response to direct, non-recurring queries such as #hashtags, URLs, domains, and keywords. The Search API (which also powers Twitter's search widget) is an interface to this search engine. Our search service is not meant to be an exhaustive archive of public tweets and not all tweets are indexed or returned. Some results are refined to better combat spam and increase relevance. Due to capacity constraints, the index currently only covers about a week's worth of tweets.