First you need to clarify what XLR outputs you are using and where exactly the audio is coming into on the mixer and that you can see metering for this input.
Please consult the manual for setting up the mixer.
Link: https://www.presonus.com/support/downloads
Find you mixer and then go to the "downloads" tab and download the manual.
If you are using the main left and right then please ensure that the audio source is assigned to the mains etc.