I would say that putting those computers on your network as close to each other as possible may help... LOL, kind of a silly suggestion but that's the best I can come up with. Just trying to be helpful.
If your network is in a building with old plaster walls, that could be a big problem for your wireless network. Those older walls will have a wire mesh inside the plaster that will mess with the signal. At least, that's what I've heard.
So... Take down all your walls and see if that doesn't help!