Dronekit, Link timeout

Hi All,
I have been working on injecting MAVLink packets using a python script through rfd900, for that I am using Dronekit library and rfd900 as my hardware for communication, the receiver side is connected with rfd900 and is running mission planner. I have python script which I am using to inject MAVLink packet, But getting link timeout, no heartbeat in last 5 seconds.
Does anyone have any idea about it?