I really don't know. I never had that happen to me. I am not sure if the nipple has bottomed out, which I seriously doubt, or the threads are compromised. When you put the old one in, was there any resistance when you were setting the nipple? The hot shot nipple is a great nipple. But I would hate to tell you to force it. Make sure it is not cross threaded, but I am sure its not. Other then that, are you using a nipple wrench to put it in? Some of them can get stubborn once in a while. Try putting some anti seize on the threads and see if that don't help.