When using Bluetooth with the CXA61/81/81 Mk II, the amplifier’s volume control takes over from the mobile devices’ volume. The mobile device then produces a Bluetooth stream of fixed level volume, which is then adjusted at the amplifier’s end.
It is normal behaviour for your mobile device not to be able to control the volume of the CXA when connected via Bluetooth. Volume adjustments can be made on the amplifier itself, using either the remote control or the front panel volume knob.