Object box checkpoints will only register if you drive through the southern most wall and out the northface.
To orientate yourself in Traxx hit the 'Home' key, this will set the mini-map pointer due north... now create and object box in front of you... the wall you are looking at is the wall you must drive through in order for it to register your truck... if it's facing the wrong way just rotate the box to the proper direction to match your road so you drive through that southface.
I'm glad that you are using object boxes for cp's since they are so much more precise than those ckboxes. :) nice.
http://malibu350.com/mtmg/traxxnorth.jpg