What is the device you’re streaming from?
If iPhone, this can be caused by the volume limiting option being set on (by default the phonak aids ‘look like’ a headset as they are not MFI).
If you go into Bluetooth on the phone and click the i against the master aid and then device type and set it to ‘hearing aid’ this stops iOS volume limiter operating for the aids. Phonak support isn’t aptly named.