I have just got my Rpi3 and set it up to work headless with no problems. I happened to have the pi plugged in within a meter of the router. I have now tried to move it a few yards away and I cannot see it on the network. Moving it back to the original spot and it reappears.
So I assume that the inbuilt wifi is at fault. My rpi2 with usb wifi dongle works throughout the house.
Has anybody else had this problem? Should I be returning the board?