Hi Ed,
Can you please provide me with a sketch of your system setup?
Usually if you are using a network with just ADN4665 Drivers and ADN4666 Receivers, then the last receiver on the link has a termination resistor. The resistor value matches the characteristic impedance of the cable.
Adding multiple terminations can lead to a situation where the impedance presented to the Driver is too low, in effect overloading the bus.
Regards,
Richard