How can i change the Rx delay after TX?

[ImanIDG]

Hi guys. I'm working on a lora end_node device (OTAA mode) with ATmega328p, RFM module and 16MHz crystal. The device send "join request" ,Instantly the server send ack massage and Gateway do its part too; but it seems device RX window can't get the ack massage and lose it most of the times. Sometimes it works but not reliable at all. I tried "LMIC_setClockError(MAX_CLOCK_ERROR * 1 / 100);" too. it seems to me that maybe changing the RX delay after TX could help. Do you have any suggestions for me?