microArranger. Not able to control acc vol from master keybd

[Laurie2]

I often play the microArranger from my Yamaha HX1 via midi. It works extremely well except I cannot find a way to control the acc. vol from the HX1 expression pedal.
I have two other keyboards which I also control from the HX1 and they do not have this problem, one is a Yamaha PSR 7000 but even the Technics KN2000 is fine.
Having tried what I think is every possible combination of midi settings on both the microArranger. and the HX1, I am beginning to wonder if this is actually possible.
Anybody able to help please