I can get a Half duplex data communication going but not full duplex
USB is a Half-Duplex protocol. There's no way to have TRUE full-duplex. 2-wires, differential levels, can only do one thing at a time.

However, with the proper buffers in place, you can make it seem like it's "FULL", in that you can send and receive at the same time from the USART. But it never truely will be Full-duplex for the USB.