I fuel as the app instructed, but my fuel balance for the trip is still negative?

This may have been due to the plotter not being able to connect to the network momentarily. Therefore, that consumption data may have been from an earlier measurement point, not exactly the last data at the time of fueling. When the plotter reconnects to the network, usually when it shuts down and restarts, it gives us that correct data. 

When you press the "fuel" button from the application, it also informs you when that data was retrieved. This is worth checking so you know if that data and consumption data is the latest.