Cubase mixdown sounds different

[luigi71cx3]

Good morning everyone.

I'm not sure that this is the right place to post this question.

I am experiencing a quite unusual situation with the mixdown in Cubase (hopefully due to my limited skills).

My project consists of a few tracks VST (Kontakt player) and a physical Virus TI Desktop (external then connected via USB). The Virus TI is seen from cubase as a normal VST, with the difference that is the Virus TI to generate sounds instead the PC. The sounds thus generated come back into the PC via the USB connection.

While listening to Cubase (playing back the song) everything sounds perfectly, but when doing the mixdown and then listen to it (whether mp3 or wav), some tracks played by the Virus TI sounds without all the voices , for example, if a track consist of an arrangement for three voices , only one voice is played instead.

To make it even more difficult to understand the problem is that if I do the mixdown only for the portion of the song where the problem appears (including few bars before and after), everything sounds perfectly , like listening in Cubase (playing back the song).

W7 x64, 8GB RAM, intel i5, Cubase 6.

Any suggestion is greatly appreciated , thank-you very much to all.