No Volume control when using HIFIBERRY DIGI+

HI, first thanks a lot to VOLUMIO team for this wonderfull developpement, I enjoy it a lot since several months.

Before I was using Analog HIFI BERRY DAC associated with my Raspberry, now, I have just replaced the analog interface by HIFIBERRY DIGI +, I have also loaded the latest VOLUMIO software revision 31/07/2017.
I have not found the way to operate the volume as usual, the volume is permanently frozen a 100 % permanently, no change to Playback option menu are able to establish volume control.

Thanks a lot for your assistance.