Question about lining pens (Copic brand)

I picked up a pack of these here: http://www.amazon.com/Copic-Markers-Multiliner-Pigment-4-Piece/dp/B006VROQP6/ref=sr_1_4?ie=UTF8&qid=1395430254&sr=8-4&keywords=copic+multiliner

It’s been about 5 days since I panel lined my old 1/144 Wing Gundam (it’s an early 2000s
kit) and I can still wipe away ink from the model with my finger. Is this normal or do the pens that I bought not work well on plastic?