Hi James,
My first thought would be insufficient mixing of the paints. I'm assuming that you're using a spray metallic black - you should shake well, for at least 3 minutes to ensure that the paint is thoroughly mixed. Even though it doesn't contain any pigment, shake the can of clear gloss for the same length of time.
My second thought is that the ambient temperature may be too high and the paint is drying before it has had sufficient time to level and form a glossy surface.
I'm not sure what's going on with your decals, but be careful applying a spray-can clear gloss over decals. Many spray clearcoats are lacquers, and can cause decals to lift and wrinkle (and sometimes split). When spraying over decals, apply light mist coats and allow to dry before applying your heavier "wet" coats.