Newbee guidance required

Firstly I am a Newbee and I am slowly working my way through this Xbee stuff so please forgive any ignorance.

I am trying to sort the best way forward to achieve the following:

1 co-ordinator to transmit to 8 endpoints simultaneously every second. The endpoints are able to individually transmit data back to the co-ordinator.

I have the set-up in Broadcast mode which is not stable enough, slow data, missed data etc.

Is there simple steps to improve the broadcast performance or should I be looking to have the co-ordinator transmit to each endpoint in quick succession through API commands. I have had a simple point to point set-up working but to expand this requires assistance.

The co-ordinator is controlled through a FEZ mini and the .net micro-framework and the endpoints are controlled by PIC micro’s.

Naturally any guidance most welcome including any API command examples for multiple nodes.