Hi Par,
The level sent to the headphones from the master headphone cue is dependent on the master volume level, so if you have the channel meter peaking at 0dB, but the master meter is low and only peaking around -24dB, that's going to be a significant difference in levels.
With the headphone cue mix knob at 12 o'clock between CUE and MASTER, set the master volume knob so the master meter matches the audio levels from one of the channels when the fader is all the way open (crossfader too if you use it).
Then, press the master headphone cue button, then turn it off, and press the channel cue button - the levels should now be equal from one to the other.
If this is not the case, a video may be helpful to help diagnose if there is a problem with the hardware.
Thanks!