@Karim > The channel trim levels should be set to around 9-10 o'clock, that would put the channel meters around the first orange LED on the meter. Next, open the channel fader and put the crossfader to that side, and adjust the master meter to around 2-3 o'clock, that will put the master meter around the same level as the channel.
Now if you press any of the headphone cue buttons, adjust the mix knob to the 12 o'clock position and turn up the headphone volume knob. If this does not work, try another pair of headphones. If that still does not work, it could be an issue with the headphone output.