According to Facebook and Instagram rules, requests for search results by hashtag have a limit. Each user can query a maximum of 30 unique hashtags on behalf of his Instagram profile within a rolling 7-day period. Once you query a hashtag, it will count against this limit for 7 days. Subsequent queries on the same hashtag within this time frame will not count against your limit, and will not reset the initial query's 7-day timer.
That means that you can have only 30 active Instagram feeds with search results by hashtag. If you have more than 30 such feeds generated, only 30 recently queried feeds will be active.
If your feed has stopped working because of the limit reached, you need to wait for at least 7 days and click the feed's link to reactivate it or generate a new feed with a search by the same hashtag.
Note: the limit is valid per single Instagram user. If you have several Instagram users, you can rotate them in the My Instagram accounts Basic setting to increase your limit.
Learn about other reasons why Instagram or Facebook feeds may fail.