Monitor buttons cut the main mix

Maybe I'm doing something wrong, but I don't think so!
I've got Mixvibes Home installed on a laptop with a USB soundcard connected. The software recognises it and it's set up as the monitor channel - however, when I click on the monitor button (the headphones icon) it cuts out the main mix.
Maybe I'm being very stupid here, but I'm used to systems where the main mix keeps playing whichever monitor I'm listening to!
The computer is a laptop, with a USB soundcard running as the monitor channel (which works using VirtualDJ...ughhhh!)
I'd be glad of any help.
I've got Mixvibes Home installed on a laptop with a USB soundcard connected. The software recognises it and it's set up as the monitor channel - however, when I click on the monitor button (the headphones icon) it cuts out the main mix.
Maybe I'm being very stupid here, but I'm used to systems where the main mix keeps playing whichever monitor I'm listening to!
The computer is a laptop, with a USB soundcard running as the monitor channel (which works using VirtualDJ...ughhhh!)
I'd be glad of any help.