Confirmed that we are having this issue also.
If we set the since_id to 0, the tweets appear in the response but if we set the since_id to the last tweet we received, we do not get any tweets that appear afterwards.
This issue started on Thursday or Friday of last week.
UPDATE:
Example of request made to API
https://api.twitter.com/1.1/search/tweets.json?q=%23hashtag&count=20&include_entities=1&result_type=recent&since_id=574033753804140544