Have just realised that there *is* a bug of some sort with this in Platinum. I've checked with the same project in X3, and it works fine, so doesn't seem to be a hardware or project issue.
The master fader on the V700 is not responding to changes within the program. So if I move the on-screen master fader, or control it with automation, everything sounds as it should, but the physical fader doesn't move.
It works the other way; physical master fader moves are reflected in Platinum. Note that this only applies to the master fader; all other faders are working fine.
A particular annoyance arising from this is if you accidentally touch the master fader, which will typically not have changed from the position it had when starting up Sonar, the current physical position is set as the actual fader level. This happens if you just slightly brush against the fader. So if the starting position was -inf, then you have silence. Totally threw me til I realised what it was.