Why am I seeing "Data source currently unavailable."?

This could result from multiple reasons, some possibilities are listed below.

No bus is operating - Between 2 am and 5 am literally no buses are out running, including diesel. Wait till the first bus of the day is out and running (normally after 5 am). Or if extreme weather approaches, AT may suspend all buses till it is safe to do so, but do check AT Travel Alerts for confirmation.
You have no internet connection - Unlikely if you can still see this text. You could do a speed test.
Auckland Transport's API is down - Check if you can access https://dev-portal.at.govt.nz/ and send a request. How to send a request using a browser?
You have entered the wrong key - Check if the keys you have entered in Settings are the same as it is shown in your account.
Other reasons - Pull a issue here on GitHub and I will have a look. Attach as much information as you possibly can.

How to send a request using a browser?

This JSON will be pretty big compared to the one app sent, so I recommend a WiFi connection.

Sign in to your developer account at https://dev-portal.at.govt.nz/.
On the top right navigation bar, click APIs.
In the list, click Realtime Compat.
On the left side, click Vehicle Positions in the list.
On the right side, click on the green Try It button.
In the pop-up panel, scroll to the bottom, and click Send.
A JSON file containing all vehicles and trains operating will be displayed on your browser, see if it contains anything bus-related (e.g. a fleet number "label": "NB5835").