Ive been on twitter since dec 08 and have asked Twitter many times to help resolve this. people ask me all the time why my tweets don't show in hashtag feeds and i really want it resolved!
Happening to me as well. I NEED my tweets to show up in the Hashtag feed for a university course. Unfortunately, Twitter Support has yet to solve this problem.
Simply saying "Due to the volume of tweets, Twitter does not index all tweets mentioning a certain hashtag, and often adds filters to only show results they think will be relevant" is not a good enough response.