Transparent inks can't stick to the surface of metallic inks.
We've recently had this problem at the offset print house where I work. We wanted to print a black and white image where all the white areas were silver and for unknown reasons we forgot what we already knew.
First we simply printed a solid silver rectangle and printed the black image on top of it.
The result was that the black image became very faint. Almost not discernible. The ink simply didn't stick to the metallic surface of the silver spot color and it didn't look good at all.
The solution was to do the opposite: First print a solid black rectangle and then print the white areas in silver on top.
Since metallic spot colors are opaque the silver covered the black entirely and the black parts was only visible in the holes of the silver image.
It's advisable (if possible) to always print metallic colors on top of black ink (or some other ink, blue for example) since it gives the metallic color more depth than if it's printed directly on paper.
Please note, that the order in which the inks are applied has nothing to do with how you layer your objects in your document. It's a decision made by the printer. It's common to print metallic spot colors last. If you for some reason want to have influence on that, you need to talk to the print house.