Dave Richardson is a SketchUp genius. He has helped me tremendously. One thing that I learned from him is that many if not most of the models in the warehouse are not drawn correctly. Their reference points (if that's the correct term) are not in the right place. He exploded a model while I watched and properly located the reference point. The piece then dropped perfectly into the design.
I'm still somewhat of a beginner, so if I have misstated any of the above, it's my fault, not Dave's. The point is, make sure the reference point for the model is in the right place. If you're beyond the beginner phase in SketchUp, don't be shy about editing the models you download from the warehouse.
Scotty: I can't change the law of physics! I've got to have 30 minutes!