Problems receiving data over USB with CDC

Hi,

I have a program set up to transmit ADC data over a USBUART connection in CDC mode.  I am having no problem connecting and sending data, but I cannot receive data.  The USBUART_DataIsReady()is simply never ready. 

Another strange thing is that when I do send a character over a terminal program, the constant output is obviously affected - it seems to slow down and appear jerky in transmission.

 Any help would be greatly appreciated.  See attached project.

Thank you,

 Tom