A couple of questions:
1. What type of press are you using (single stage or progressive)?
2. According to your above comment when you back the die off one full turn you are not getting powder thrower actuation. The Lee instructions say to back the die off 1/4 of a full turn from the shellholder, not 1 turn.
3. If you expand the case mouth without the powder thrower but instead use the orignal top plug on the expander die and the die backed off 1/4 turn from the shellholder do you get sufficient case mouth expansion?
If the answer to #3 is "yes", but the same setup with the powder thrower does not give you sufficient case mouth flare then a possible solution is to either carefully grind (or preferably turn on a lathe) a few thou off the mouth of the expander die (make sure to keep it square). That will allow the die to be turned down further into the press and let the expander plug to go further into the case and give you more flare.
