LeBlanc on BB

I'd throw my Mark on this miss fortune, and then I would jump to her and detonate it, however, she wasn't taking more than 1/4th damage when I had void staff and rabadons, now you may look her up and say she had Banshees on, but thats just it, void staff is supposed to give magic pen, and I had rabadons, and this is without the Banshees Veil passive up. And I know I was detonating the mark with my W. Is it a bug? Or is LeBlanc garbage damage now too?