I'm putting in a solid bet here and saying that the two routers both do NAT and that your ettercap is running behind router B (on laptop B). If that's the case, try sticking it behind router A (i.e. hardwire it) and try again.
Also, hit up wikipedia, look at the basics of networking (specifically address resolution protocol) and ethernet. The answer will be extremely obvious after you have got a grasp on what you are trying to do.