Are your monitors powered as in the amplifier is built into the monitor and the monitor
requires that you plug it into a power outlet or are they passive and require a separate amplifier off the stage somewhere and connected to the monitor with just a speaker cable?
Actually what are the monitor models?
Giga had a good suggestion on swapping a good sound monitor with one that is having issues.
There are two eq’s on every mix output, parametric and graphic and as mentioned the mix send pick off points, lots of places there to completely change the sound of the monitors.