Sorry – don't really understand. You say that the volume is being cut. A few questions (please answer them all in full):
1) Does this occur with a "Through" preset ?
2) Is it the same on both channels ?
3) When it happens, do the various level settings show 0dB ?
4) Does it occur on both jack and XLR ?
5) If you wiggle the jacks, does the level change ?
6) Do the output meters show this gain change ?