Sound reducing when playing with accoustic guitar

Hi all



I recently (2 weeks ago) bought an Elevenrack and yesterday i've spent some couple of hours with it. While playing with my electronaccoustic guitar I've noticed that after 10 to 15 min the sound of the guitar (from main output) reduce.



For your information, the guitar was plugged to the true Z, and when this happened all the volume controls didn't change. I mean I didn't notice any phisical or softwere volume control changes. And I was playing live no protools for this test. I've used the main outputs and connected them to a PA.



After restarting the unit the volume comes back to normal and then again the same thing happens.



Please advise me

Thanks for your answers