Ground Station and Air Station not connecting

I am using 3DR Single TTL MINI Radio Telemetry 433MHz 250mW for PIXHAWK and APM for wireless communication between Ardupilot APM 2.8 and mission planner running on laptop. According to the LED status, It should show solid green when connection is established and blinking green when searching for connection.

Whenever the ground station is connected to the air station, we face different situations.

case 1 ) solid green light is established and after few minutes due to loss of communication, the green light continues to blink.

case 2) the blinking green light is observed, the communication between two telemetry ports isnt established in the first place.

What could be the possible issues?Why isn’t a solid connection being established?