Make sure your A4 is highlighted on both sections of the C6 connection panel and then press OK - you get to the connection options via a button on the C6 screen, but unfortunately I’m not in front of my computer right now to tell you which one. It should be obvious, though! Also, make sure you click on the file(s) you want to transfer in the main window before you press send, so that they are highlighted in blue. Anything unhighlighted won’t be sent.
Those are the 2 most common things to overlook.