unity3d – Image target can only be detected in a limited field of view on hololens 1

I am a beginner using hololens1
I am developing a simple AR apps to detect a target image and display an object.
I am at a stage that I can successfully run the app on hololens and detect the image target and displaying the desired object. I am using unity 2019.4. Vuforia 9.8. MRTK.

However, I find out that the field of view that I can detect the target image is very limited (only a very limited space just in front of where I started the app). When I turn my head to somewhere else, even if I move the image target right in front of the camera, the object cannot be displayed. It shows in the log as extended tracking. I believe that there is some setting that limit the space where the augmented reality content can be displayed but I cannot figure which setting I need to adjust. Please advise. Thanks a lot

Expected to show augmented reality content whenever the image target is detected