What are you using to neck size with? If you are using a FL sizing die and just not taking the case all the way into the die to neck size, realize that you are probably moving the shoulder forward. If this is happening, you are reducing your headspace and thus the added force to chamber.
If you don't understand why the shoulder moves forward, I would be glad to explain.
I'm using a Lee Collet (neck size only) die. I don't understand exactly what you're talking about (neck sizing by partially using a full length die). I would ask you to explain, but I take it that this is not proper technique so I don't care to use it.