How to lock object when get tracking

i have a project that is augmented reality project. its about to show 3D human ar when the target marker with some screen capture my problem is when the device targets the marker, my object show up but

1. my object not stable always get a shake

2. when i get far my object gone, because that is human object i must go far to get the full body

3. i want that object get lock so when i turn around i can get the object in another side without focus to the marker

please help me for this problem thank you