just tried a FV-500H Expression Pedal
-used a stereo cable pedal "expression out" to kpa pedal 1 in
-under 'system " page at "PEDAL 1" pick : Expression (type 1) Wah wah
-under stomps pick "A " and pick wah wah
manual 3.4 peak 5.1 pedal range +22% Peak range +42%
bypass@heel 100% mix ducking +0,1 volume +2.2
Its working without any problems by me that way
Perhaps your stereo cable is not o.k. ?
greatings Dorrus
Dorrus