A/C Zones on Thermostat

I have three roof top units and three zones indicated on my thermostat. My forward and rear units (Zones 1 and 3) work fine. On my thermostat, when I select zone 2 there is nothing to set, it's a blank screen. I was told a couple years ago that this was because it tags off the zone 1 unit. I don't think that was a proper answer and was asking this forum if anyone has any ideas. In short, I can't operate the center unit. Thank you in advance.