Robert, I reload several .17s, including the Hornet. Never had a problem. Suggest a call to RCBS Customer Service. They will walk you through the setup and if it doesn't work, I'm SURE they will replace the sizing die. My .17 HH dies are also RCBS.
However, as Darkker suggested, you might have the dies mis-adjusted. Make sure the decapping rod is not bottoming out in the case. Remove it (for grins) and see what happens. Could be an oversized / wrong expander ball. Hey, nobody's perfect.

Let us know how you make out.

BTW, sure wish I could find some Lil Gun!!! Seems to the THE powder in the .17 HH....

ron