Are these two things correctly understood by me?

1. your console has two reference levels on their inputs, +4 dBu and -10 dBV
2. you are ALWAYS using +4 dBu on the console inputs.

This means, in other words, that you are making the inputs of your console less sensitive.
By this, it can support hotter input levels like one of the typical studio levels, in your case +4 dBu, right?!

If the input level on your console is too low (how low is peak/RMS?), then the signal coming out of the HW output of your UCX II is low, and this can have several reasons.

Check the signal level on HW inputs and SW playbacks that are sending audio to this particular HW output.

Ensure that you are in submix mode, click to the particular HW output that is connected to your console.
Observe metering of HW Inputs (top row) and SW playbacks (middle row) and check the fader position.
You can additionally press the button "sub" on the right side then it shows you only those channels of top/middle row, which send audio into your HW output/submix.

Very / too low HW inputs? Check fader position (for this submix). Check reference level, maybe too high and makes input too insensitive to support hot signals .. check manual which reference levels are supported. You can add digital gain if needed, if you have consumer level signals.

Very / too low SW playbacks ? Check fader position (for this submix). Check the application which is sending audio to this SW playback channel. Maybe you turned the volume knob of a music player or DAW down?