Hi Aidan, I had a similar problem with my SK2 used as a master keyboard (Fixed velocity) It was the "sounding point" set to Auto ? affects the External zones?
From The manual:....
Page 81... SOUNDING POINT (S)
Set the sounding point if the internal keyboard is played.
DEEP All the section sounds on the deep point in the stroke.
AUTO The Organ section sounds on the shallow point if the [ALLOCATE] of the Extra Voice section is
OFF.
NOTE: The External Zone notes on at shallow point, and the velocity value is fixed at 100 when
the Sounding Point is set at AUTO, and the [ALLOCATE] is OFF.