Are you doing stroke > flash > stroke ? (not sure if that is how professionals call it, SOMEONE CORRECT ME PLEASEE ) 
Is it opaque ink.
I had the same problem, printing in black with yellow ink.
So I bought opaque ink, two strokes, and it works just fine.
Is it opaque ink.
I had the same problem, printing in black with yellow ink.
So I bought opaque ink, two strokes, and it works just fine.