I have an older trackable that I created back in Jan 2012 that works really well.
I decided to add some more images to the trackable, but my image was missing from "My Trackables" so I added it again and added a few more.
The dat file it produced now doesn't work as well and at distance the model now sinks below the physical marker disappearing. The back egde of the model essentially becomes occluded.
Thanks!
Try extending the far clip plane of your ARCamera.
What other issues are you seeing w/ the target, regarding it not working well?