Midi input slider volume scaling

best sunhouse team.
first off, really love the sp2 system.
but my question is i’ve got a behringer bcf2000 and when I want a slider to set the volume, the software will then set the volume very fast high in the beginning, slowing down to the top. am I doing something wrong? If I use the slider for left/right channel it just works fine… its like the software uses a multiplier of somesort (amplifier)
i’ve searched the forum but couln’t find my answer
thanks in advance