How did you fix that? Been kind of having the same trouble with one of my projects
Which part? The collapsed shoulders? Or the messed up trimming?
My process for a perfect neck down is as follows:
One shot lube outside and inside neck
Wilson bushing die with a bushing .010 size step down smaller than original parent case
Wipe down then reapply one shot
21st century neck die with another .010 step down to get rid of the unsized portion of the neck (donut) and in my case set desired neck od.
(This die sizes all the way down to the neck/shoulder junction unlike normal bushing dies )
Wipe down neck area again because of brass shavings
Thoroughly apply graphite inside the neck
And use 21st century expander mandrel 1thou larger than desired neck id/neck tension because of excessive spring back.
(This is where I got collapsed shoulders at first, from not using enough graphite, and trying to go to fast, you can feel when the expander doesn't want to go. Slow and steady is the way.)
Then run it on a Henderson trimmer because most of the cases have stretched from all that brass movement.