Make sure to point your phone to the QR code or, if you set an image target, to the image you uploaded.
It might be the case that your 3D assets it too small or too big to appear in the camera view.
Scale the model by pinching the screen with two fingers. If that does not work, change the size of the model by adding a metadata key named
scale to the 3D models. See how to add metadata in the Data Page section of the documentation.
Now rescan the QR code that rebuild and refresh your AR.js app.