Did you Anneal the brass? At times this will allow you that little extra you need.

Try one piece of brass. Have a bucket of room temp water available. I hold the brass with my fingers at the rim, heat the upper (mouth end) of the brass back about 1/3 with a propane torch turned up to heat quick, when to hot to hold just drop it in the water. Then blow off the water on the case, liberally coat with Imperial sizing wax and run it into the die.

Let me know how it works out!!
Don