Dave Smith Instruments SYNTHESIZERS

hey, 
so i've had an oxygen for a few months and recently purchased an evolver.... but I 
have never tried to use the aftertouch features of either until today... and I can't seem 
to get it to work.  I can change the value of the aftertouch by pressing MIDI/SELECT 
on my oxygen, presing aftertouch, and then moving the data slider, and the evolver 
responds to this.  But if I just press and hold a note on the oxygen and then change 
the pressure, nothing seems to happen.  What am I doing wrong?  Is there something 
weird I have to do with the oxygen?  Do I have a clear concept of what aftertouch is?  
It's supposed to be the pressure applied to a key after the original note on/velocity 
message, right?  Did I get a bunky oxygen?

thanks
mark