How to manage sleep devices

I am using Series 2 pro module of 2.4GHz. One network has 1 Coordinator, 1 Router and 10 End- Devices. My problem is every 1 min my end-devices wake up and send data to Coordinator. There is loss of frames and also the range is less. Can anyone help me out on how can I manage the timing where I will not have frame lost and also why the range is varying in same location on different days

As for when you send the data, that is controlled by when you send the data to the radio to begin with. Controlling who wakes up when is based off of your SP and ST time frames.

As for the range issue, what is the environment and part numbers you are working with?