When you are playing long techniques with most of our orchestral libraries such as BBC Symphony Orchestra or Albion ONE the dynamics (how loud/hard the instrument is played) is not affected by the velocity (how hard you play the keyboard).
Instead the dynamics is controlled by MIDI CC#1 - this is usually the modulation wheel found next to the pitch wheel on most MIDI keyboards, or by moving the dynamics fader in the plugin window.
Short techniques - such as staccato or spiccato - should respond to velocity in most libraries by default.