Troubleshooting steps:
- is the gain up on the channel you're previewing (does it show levels on the mixer)?
- have you selected the cue for that channel?
- is the cue mix knob turned to the middle (or to the CUE side of the mix)?
- is the volume turned up on the headphone knob?
- do your headphones work with another source?
- is the source on that channel from software or hardware?
- have you tried it with a hardware source connected to a line input to see if maybe the problem is your software output configuration?