I've found this happening when I operate the CSR while being online with the amplifier. If you try to control from both ends they can get de-sync. Then CSR volume slider will have no effect on the amplifier, the only way to control volume now will be from CSD.
To fix this you have to set the slider on CSR to minimum and then manually set the volume on CSD to -60. Once you do this they'll both be at sync again.
I think the problem is that in CSR the minimum volume value is -60 and in CSD the minimum shows as -∞