I agree with some previous replies that it is probably due to the packing, but there is a little more involved.
When rolling the nose, even if you THINK you are doing it perfectly, there is always the possibility of rolling one side more than the other. This is usually the result of using the same hand (probably your dominant hand) to roll two separate directions. It just isn't even. This is compounded by the speed of the opening. If you are having end cell closure, I'm guessing the openings are fairly slow. (Just a guess)
Try this...don't roll the nose at all. Pack as normal and just push the nose in as you described but without rolling it. I know some people are afraid of doing this with Sabres, but I have about 400 jumps on a Sabre and I never roll the nose. (I am loaded about 1.1) I have consistent soft, on-heading openings.
Let me know how it goes.
Rock