As someone who works hard to makes designs and building instructions easy for everyone, it's frustrating when users---even members of this blog---say that they can't get a model to work. I can't tell you how many people complain to me that they can't build the models found in various highly-rated NXT books (or free online).
I've had the same experience---or have seen my family have the same experience. We KNOW that the model works---and it has been demonstrated on video---so why the disparity?
Does varying part wear make that big a difference? Is there a difference in the production of the same part? I'd like to gather some of your insights here for the benefit of our frustrated builders and designers.