HootSuite announced on their official blog that Twitter's API is currently experiencing a critical issue, and that is the reason of which some third-party clients (including HootSuite), have some problems. They said that "Home" and "Mention" streams are currently not updating correctly, and this also affects the users of iOS and Android. Additionally, some users may also experience crashes when they try to view certain streams.
They also said that the guys from Twitter are aware of the problem, and they are working in order to fix this.
You can view the current status of Twitter's API here.