I am absolutely frustrated with this problem. It still happens randomly even with two simple imported wav files. I thought it was the US-600 VST driver, but even with the ASIO DirectX driver I get the same results. Something is really wrong. Exiting Cubase and restarting will make it work once in a while, but still, even if I get two wav files to simultaneously play, if I import a third one, it is guaranteed that the newly added one will not play. Grrrr! This has rendered the software completely useless to me 
Any help is appreciated!