can't hear myself in my headphones

[fiddlelaura]

I having been attending informal tune sessions with friends and mostly things have gone smoothly. My problem is that I cannot hear myself in my headphones. I can hear all the other participants, but not my own voice. I have fiddled with volume knobs and various sliders, and I've jiggled cables and changed my adapter plug, to no avail. My solution has been to put the headphones on one ear only so I hear myself in "real time" while everyone else is in my other ear.

I have a Presonus USB96 audio interface, with Bose headphones that I already owned, and Pyle PDMic78 microphone.

Thanks.