ESD Mats failing with constant monitoring system
I'd guess that the system MAY work by determining that a capacitor (that's YOU) was connected from ground to mat.
If there is excess leakage from mat to ground directly rather than  via the management system the body capacitance may be being swamped.
Adding two bodies directly to the mat may add enough capacitance to fall within expected range. 

The mat/station mix makes less sense BUT a possible explanation is that location 1 is of low resistance to ground - and MAYBE damp.
Any mat there is more likely to have an unintended path to ground and moving damp-mat-1 to location 2 carries enough may to surface conductivity to cause a fault. 

Test: Place a sheet of dry plastic under the mat in station 1.
If my arguments are correct it "should" remove the fault condition. 

Report back.