Perhaps a small amount of honing can enlarge the chamber just enough for easier extraction? The chamber should be conical and accurately made, but who would know in a derringer? It's not for long range shooting, and you aren't going to remove more than 0.001" of steel anyway. Here is something to read about the process. It's not like you can make it worse... unless you plan to ship the firearm to the manufacturer again, or to contact a local gunsmith.
I really don’t want to invest any more money into it. Thank you for the information. I have bookmarked the page and will try it.