I’m guessing that you have your headphones plugged into your iPhone/iPad? if that’s true, that’s not the intent of the QU-You App. Think of the QU-You App as another Remote Control to your QU mixing console. It allows you to move faders on the console, without having to be at the console (i.e. it allows band members to mix their own monitors). So that’s all it is for is to make remote fader movements only. There is NO audio sent from the QU console to your iPad/iPhone.
Audio is only sent OUT of the various outputs of the console… or out through a remote dSnake extender box if you have one of those. So any console output that has an audio mix directed to that output, can have it’s audio contributors “mixed” at the console manually… or “remotely” using the QU-You App (or QU-Pad App).
If you want to use headphones, you will need to setup a headphone AMP that you can feed your various QU console MIX and now potentially GROUP outputs into. Then you can plug your headphones into that headphone AMP, and presto… headphone monitoring!