I took another look at the pic of the brass and to me it looks just like what happens when you push the shoulder back to far.

Try this when setting up your FL sizer die;

Use one piece of brass for the initial setup.

If you have a tool that accurately measures your shoulder bump take that measurement first. If not you might want to look at this tool.

www.larrywillis.com

Install the die in your press per the instructions that came with the die and back it out another 1/2 turn. Lightly lube the body of the brass and run it up into the press and inspect the neck with a good magnifying grass. You should be able to see how much of the neck is being sized "stop ring" which should be somewhere around half way down the neck. Keep adjusting the die "downward" in small increments until the neck is being sized to the point "before" where the neck just meets the shoulder. Take another measurement of the shoulder and the brass should have grown a little. Make another adjustment to your die and take another measurement and keep making adjustments until your only bumping the shoulder back .001 - .002 and no further. Each time you run the brass into the die check to make sure you have enough lube on the brass so not to get it stuck in the die.

This should end your problem.

Bumping the shoulder back too far would also have an effect on your neck tension. If you don't have any or minimal resistant on the case neck running over the sizer button this is a big hint your not sizing properly. When the die is set up correctly, you should have a fair amount of resistance. To minimize this you could lube the inside of the neck though I feel it is not neccesary if the inside of the neck is cleaned properly. I use a brass bore brush on my RCBS case prep station. I also wrap "000" steel wool around the brush so it fits tighter in the neck. This will clean the powder residue from inside of the neck and the brass will size like if it were brand new brass. There is a stainless steel media tumbler made that will do as good of a job. I plan to buy one after the holidays because the whole unit will cost $265 but I've heard it is the last brass tumbler I will ever need to buy. The media will last forever!

http://www.stainlesstumblingmedia.com/

Good luck!