Positional device tracker works but not the trackable marker

Hello,

I have made an AR App (on Unity, for an Android phone) with an image target. The image target I chose is a QR code in order to have a lot of features.

My problem is that the AR detection works sometimes right away, and sometimes I have to relaunch the app or move away from the target several times in order for the target to be tracked.

I am using the positional device tracker and extended tracking (on Unity 2019.4).

What I don't understand is that the positional device tracker status always detects the image target right away (status = "tracked - normal"), but the trackable marker does not systematically ("no pose-unknown") and that is why I don't have anything rendering at times.

Would you have any idea here why the marker is sometimes not detected?

Thank you in advance