You will probably just need to add more Zigbee router devices as Zigbee relies on mesh technology and really depends on you having many Zigbee router devices to extend the network range and coverage.
You also have to understand that Zigbee technology uses low-power radios that send short data messages at low data rates (so messages that do not come through will need to be resent), and using 2.4GHz means that signals from and to Zigbee devices has a relatively very short rang and very poor building-material penetration power, (as well as generally bad signal reception) and thus the solution (other than doing your best to actively avoid interference) is just to add more Zigbee router devices, preferably buying some known-good Zigbee router devices to work as dedicated always-on Zigbee repeaters.