Tweets occasionally appear on the Twitter homepage before they are available to be shared on other services such as Feeds. Unfortunately, the reason for this is a result of how Twitter's post-searching tool works. A tweet must be indexed in order to be made findable by a search. Twitter only indexes and allows you to fetch with the API a percentage of the total tweets that are posted, however.
Since Feeds relies on the Twitter API to fetch content, the end result is that sometimes content that appears on, say, Twitter.com will not appear in searches, even searches that are an exact content match.
Twitter takes a lot of factors into account when deciding what tweets to index, but it seems one of the major ones is time. If it's been a while since a tweet with a hashtag has been posted, it most likely will not show up in Feeds. For example, check out this search on twitter.com. There are lots of posts with that hashtag, but it doesn't work when you try to enter it into Feeds. As of this article's writing it's been at least 5 months since the last tweet with that hashtag, so Twitter doesn't think it's worth indexing.
So what should I do?
Well, unfortunately there is not much TO do. We can't force Twitter to start indexing more tweets.
You can always manually add posts to your feed. Click "Social Media Sources" on the Feeds Moderation Console, then click the + button next to your existing twitter hashtag source. Copy and paste the URLs of the tweets you want to show up. You should only have to do this the once.
Otherwise, this may be a blessing in disguise. If a particular hashtag is not very active, then it's probably not doing much for you in your Feed anyways.
For now the best thing to do is look for alternate hashtags, or wait until that hashtag becomes more popular.
Also make sure you are doing everything described here: https://support.twitter.com/articles/66018