I would measure the expander on the sizer dies. RCBS gets this wrong sometimes.

I had a simular problem way back with a 300weathby die.

You can do several things. Take a fireformed peice of brass, and measuer the inside and outside neck distances. Then take out your expander ball, and size two pieces, and measure that. Then put your expander ball and measure that. Take your 225gr bullets and measure the diameter. Give these numbers to RCBS tech support.

Are you FL sizing or paritial FL sizing? I found out the other day partial FL sizing can make your shoulders grow past the point of a fireformed case.