Thank you very much for your answer.
Now, I am not quite sure which sleep timing setting should I correct.
End devices sleep during 1min before polling for any command.
xbee coordinator sleeping time settings are:
SM: no sleep
SP (cyclic sleep period): AF0
ST (time before sleep):1388
SN (number of cyclic sleep periods): FFFF
ET (child table timeout): 2 minutes
SP=AF0 means that xbee keeps commands during 28 seconds which is lower than 1min. but my application keeps sending commands through xbee during 10 minutes, until it gets an answer from end device.
On top of that, child poll timeout=3xSPxSN is at the maximum: 2 months. So, end devices shouldn't leave after 2-3 days.
Does it have anything to do with NT (node discovery backoff) ? I have 3C x 100ms for this setting.