I believe that Soft Altitude Hold is only active when in Alt Crz and Managed Speed and I think it needs to have transited to Mach.
When you fly short relatively low sectors (below FL300) e.g. LHR MAN or even AMS the 320 doesn't go into Mach, it stays in speed (although in managed speed). Therefore Soft Alt Hold is not active with Alt Crz and Managed Speed.
In TX it is useful to simply pull the speed knob and use selected Mach, this takes it out of Soft Alt hold and the A/T will target the speed much better. Select .76 if it gets worse. In Soft Alt the speed can wander around a lot before the a/c reacts.
I press the mach/speed button sometimes as well to nudge the speed back a few knots rather than a whole mach number.
Last edited by wings11; 4th Jul 2012 at 14:48.