How can I send data to an end device that periodically enters low power leep mode?

According to the manual of Xbee-Pro ZB RF Modules, when an end device enters low power modes, its parent should buffers incoming messages and send it to end device as end device wakes. I have tried to send a test data to an assigned sleeping end device from the coordinator, but the data failed to reach the end device even the end device wakes in few senconds later. What should I do to let my data succesfully delieve to an end device that periodically enters low power sleep modes?
(In XCTU firmware setting, the function set is configured as Zigbee AT mode for product XB24-ZB .)

First, set the DL and DH of the transmitting module to the SL and SH of the receiving module.
Next, set the SP and ST time on the transmitter to match the SP and ST time of the End device.
Third, send One and ONLY one data packet. Once the End device outputs the data, then and only then can you send the next packet.

You may want to look at using the WH command.