A content source that has finished processing still shows up as "Running" in the UI despite clicking Stop. This prevents processing anything else on the content source.
View the different values for the “status” collection at MongoDB Collections Description.
You can also run a query in Robo 3T (formerly Robomongo) in the Content Source database to view the status of the last crawl:
db.getCollection('status').find({}).limit(1).sort({$natural:-1})