Setting up a 485 network
Use TVS diode and make sure the capacitance is low, you didn't put any part numbers on those zeners so there is no way to check the leakage and capacitance. 

The next problem is you don't need the pullups and pulldowns on each device, only on one end. The rest only need a 120ohm resistor. The cable also needs to be matched to 120ohm. 

You might have a voltage drop between devices and with the pull ups and/or pulldowns you are getting too much current for a given transmitter. 



Intefacebus.com has a good schematic for reference on how to terminate the buses.