Singers seem to complain about a bad mix more than others. This is often for good reason and I can relate since I sing as well. Nothing worse than having to sing without proper input to stay on pitch. Being forced to over sing is a horrible situation and can lead to physical issues. My first attempt to cure this situation is to 'force' the stage volume down. Most times, the other band members won't comply. Not much to do in that case.
My standard answer for the prima donna singer that complains for no good reason is "I'll add volume if you add more talent"