Networked computers not visible automatically

I have two desktop computers and a laptop networked together by Ethernet cable. My main desktop computer is connected to the other two through my BTInfinity Hub and then through a switch in another room. I am finding that computers are not showing in the network maps when first switched on and then I sometimes have to do a restart before they will show in the map. I can ping each of them successfully. Why do they not always show immediately in the maps.

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