I am using a spatial target for the experience. Now, When I zoom the model, only the model zooms but the 3D images placed around the same are not zooming.
Let me know if there is anything can be done to address this.
Flickering sounds like you are loosing tracking. I would try a different environment, maybe more light, or outside. You could also try a different device. Even as an Android devote i have to admit that newer Apple products seem the track the best.
I am sure the device may not be the cause as I am using I-pad Pro 10.2.
Let me check in a different environment with a better light.
What file format are the images? I've seen this happen where a JPG image won't scale, but if I convert it to a PNG, then I can scale using the gestures.