OK, that model does have a sensitivity adjustment and also has a test mode. In test mode the light will flash when motion is detected. So if you put it into test mode, you can see if the issue is that the sensor is not detecting you or if the issue is that the sensor is detecting you but your smartthings network is slow to process the detection message.
In other regards, it’s a standard PIR sensor. So shoulder height 90° to the person is going to get the quickest response. Or anything down to knee height. You want to get the body mass traveling across the detection zone.