Why is my metallic Sharpie drawing thinned out and transparent lines?
The thinness you are seeing is that your pen is dispensing more of the inks suspension medium than its pigment particles. The "ink" in metallic Sharpies is made up of particles suspended in a medium. It's the separation of particles from medium that is causing the "transparency" of your inked line. Over time, as the "ink" sits stationary, the heavier particles settle to the bottom and the lighter elements float to the top. Think of it as a paint can that has been sitting for a long time and needs to be stirred before use. In your case it sounds like your pen has been sitting in the same position long enough for the pigments to have separated and settled. Before using your pen try shaking it for a few minutes. The shaking with help re-mix the components and the pen will dispense the right amount of pigment suspended in medium based on the pen's design.

"Shake it like a Polaroid picture" before use.