There is a known issue currently where a number of non-HSO sustaining instruments (particularly sax and choir sounds) don't respond to dynamics.
There is a workaround for this which we've implemented for the next update, but you can also apply to your own projects right now.
In Play -> Expression Maps Setup, create a new Expression Map called 'CC11 Dynamics', add the 'Natural' technique and set the Volume Dynamic to 'Control Change: 11'
Now in the Rack press the gear icon and for any channels that use a sound that isn't responding to dynamics, set its expression map to 'CC11 Dynamics'
We are working on support for VST note expression which should give better results for patches that support it, but CC11 should work quite well for a large range of VST plugins.