openthread FTD child not receiving UDP message

Im using openthread in my project. the project consists multiple devices connected to boarder router(RCP) in raspberry pi. I have script in raspberry pi which get the rloc of devices in the network and send some data to devices using  (rloc ip of the device). the script working fine and devices recvie the data. how ever when the the role of device is child it the message did not received until the device change the role to router! 

why this happen when device's role is child ? or I cannot use rloc ip to send to child?

all devices using nrf52833, with sdk is 1.9.1.

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