Malfunction Of Zigbee

Hello, I have made a project using zigbees communication through RS-485(2-wires), three different master zigbees of different network mounted on single box. The project have completed but suddenly two days ago communication of one of three master zigbee didnot work properly and increase the communication time from few seconds to 4 to 5 minutes , I replaced with new one and issue resolved, but now second of three master zigbee is behaving the same as last malfunctioned zigbee was doing.

I am using Fatek CMZBR zigbees and fatek PLC to communicate. Zigbees have been mounted 30ft above in a box from the PLC panel. Master Zigbee1(Controlling 20 slaves zigbees) Master Zigbee2(COntrolling 14 slaves zigbees) & Master Zigbee3(Controlling 7 slaves zigbees).

I am unable to trace the fault, kindly help me to trace what is the fault?