This may happen when one of the radios has a different configuration, this can be solved by resetting both radios to default.
 
To reset the radios to defaults using Mission Planner:

  1. Connect the telemetry radio to the computer, select the serial port and 57600 bauds, don’t hit connect yet.
  2. Go to Initial setup, optional hardware, 3DR Radio.
  3. Click Reset to Default.
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    To reset the radios to defaults using APM planner:

    1. Connect the telemetry radio to the computer, go to initial setup, 3DR Radio.
    2. Change the link settings selecting the serial port for the telemetry and 57600 bauds.
    3. Click Restore Defaults.
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      If you are not able to establish a connection after executing this procedure, try to re-install the firmware of the telemetry radio as is shown previously.