Dave Smith Instruments SYNTHESIZERS

hey
so, i've had an oxygen for a few months and purchased an evo somewhat recently... 
but until yesterday I haven't tried to use the aftertouch feature of them.  i can't seem 
to get them to work.  I don't think it's the evo's fault because the pressure mod 
destination will respond when i type in an aftertouch value on the oxygen (by 
pressing MIDI/SELECT, AFTERTOUCH, changing the DATA ENTRY slider, adn then 
MIDI/SELECT again), but it doesn't seem to respond to when i just press and hold a 
key and then change the pressure i'm pushing with (this is correct concept behind 
aftertouch, right?).  What am I doing wrong?  Do I have a faulty oxygen?

mark