Slider won't control source volume

[LeonardoCT]

Hello, i have a custom build arduino based midi device, when i press the "listen continous" button on the plugin, i can see the value update in real time when i move the slider but when i select the audio source i want to control, it doesen't work, and i can'understand why, here is a screenshoot of my configuration:

[NeWFieKaNaTa]

Having the same issue. Other than my slider will always automatically changes to fixed mode