You might have noticed that the background colour on the header sometimes changes, seemingly at random.  Except it isn't random, and this is what you need to know:

This is what you will (hopefully) normally see.  You have connection (wifi or mobile data) and all your data has been sent to the server.

This is what you will see when you have no connection, but so far as we know, it doesn't matter because you don't have any data for the server.  Of course, the server may have some data for you, so it is best to search out signal periodically.

We are online, and in the process of sending data to the server.  If you have a good signal you should only see this flash up quite briefly.  If this colour persists then something has gone wrong - exit the app and restart it when you have a good signal, and the updates should be flushed within a short time (which should give you a light grey background).  Another thing you can try is to switch on aircraft mode and then switch it off again.

This one, as you can probably tell from the colour, is the worst.  We have some data for the server, but cannot send it.  If we don't find a connection soon alarms will start to be raised and people will start making phone calls to see if anything has gone wrong.  Try and get a connection.