What should I change in the zigbee network in order to eliminate tx failures?

I have a zigbee network with 1 coordinator and 15 endpoints. I have tx failures.
I added one router into the network. So 8 endpoints are connected to the router and the rest of 7 endpoints and the router are connected to coordinator. In this last config I have more tx failures.

How should we configure a network with 15 endpoints? I need to have 0 tx failures.

In this deployment we have xbee2.

Try switching to a polling method or Decrease the rate that you are sending the data.