I have six end device xbee radios but can only connect to a maximum of four at once.

I can connect successfully to all six devices individually and to a maximum of four at once, but when five or more are powered up, none of the devices respond to messages. When I power two of them off the devices begin to respond again.
Is it only possible to have four end devices working at once under this firmware? I am using api mode with escaped characters on coordinator and end devices.
Coordinator DH is 0, DL is FFFF, MY is 0
The six end devices have their MY addresses set as 2222, 3333, 4444, 5555, 6666, 7777.

Thank you, Nick