The N2 bus is loosely based on the RS-485 standard. The design of the NCM allowed it to compensate for power drops, bad wiring practices, and interference without the need to add a repeater every 32 devices as indicated by the RS-485 standard. The other situation that could cause similar symptoms is that repeaters, like any electronic device, deteriorate with age and could fail to provide the desired boost to the N2 network.
Measure voltage drops at various points on the N2 bus and insert or replace repeaters where indicated to guarantee reliable performance of the bus.
Our integrator partners have had good experiences with the B&B 485OPDRI. See http://www.bb-elec.com/Products/Serial-Connectivity/Isolators-Repeaters/Optically-Isolated-RS-422-485-Repeaters.aspx