First off, we are still on ST3 but will be updating to the latest ST here very soon. However, I have not had success with this feature creation since the early days of SE.
Now for my question. I have created some copper that will also have a mirrored version of it. However, I have never been successful at the mirror feature in the assembly mode with copper. I was talking with one of my co-workers recently about it and he just happened to go back to his desk and try the same thing on his computer. He was successful at the same function that I was trying to do. When I do the mirror steps, and the last step before "finishing", I see the copper mirrored. However, when I hit the finish button, it pops up with the error: "The part cannot be placed. Make sure it is not a feature library part."
I compared his settings to mine and they are the same.