go with a much shorter heel. the heel drags on the edge, causing the roughness. you should try a heel that's very short. maybe a half milimeter or so, i cant tell from the foto, but you may also be going too deep into the metal. not sure why it's a problem with copper and not in silver. i assume both are about the same hardness.
for push work, you always want your gravers to be absolutely sharp. graver control is much easier with sharp tools.