Mine is also a Logitech Extreme 3D Pro (Twistgrip), but for my setup, the speedbrake is mapped to one of the three buttons on the CH Pro Throttle. Maybe there's something with that which allows the speedbrake to work?
I hadn't thought of this until now, but where is the speed brake switch in the sim - and where do you have it programmed to - on your own throttle as well.