Periodically, TFL’s Countdown service goes down for maintenance. This is understandable since the service is still fairly new.
Usually, the Countdown system becomes unavailable on Sundays, so please be aware of that while using the app. In fact, today (February 5th), the system will be going down at 19:30, and will probably be inaccessible until Monday morning.
Please do not leave negative reviews about the app if TFL has taken down its essentials services for maintenance (e.g. the journey planner API or Countdown). You can check the status of Countdown here.
Note: If the app complains that TFL’s servers have been inaccessible for many days, please contact me. The format of the website might have changed, and that will affect how I pull data from it.