I have been engraving metal business cards for us and for a client, both with 0.21mm and 0.45mm thickness.  Most of them engrave beautifully with no problem.  But, I've had some cards, mainly the red and purple ones, but sometimes other colors, that don't engrave totally.  See the image below.  Has anyone ever seen anything like this?  If so, how did you resolve this issue?  Thanks, in advance, for your help.