There seems to be some talking at cross-purposes here.
ATNotts seems to be talking about a wifi scenario, while gemma10 seems to be talking about networking over power cabling.
Wifi signals are line of sight, with some ability to penetrate solid material (they are radio waves). If the LoS is blocked by sufficiently thick or dense material the signal will be blocked.
Assuming Windows laptop, run netsh wlan show networks from a command prompt to see what wifi networks are available, their channels and the strength of the signals.
You will also see if there are other competing signals that are swamping the desired wifi network.
SD