I am far far from being really knowledgeable on the subject but I do know that radio waves can be very directional and a foot either way from 'direct line of vision'', from transmitter, can make a difference between receiving or Not receiving a signal. One can move in front of a window to a position whereby a corner of something, such as another building, can come in the pathway of a signal. Furthermore, one might move to a position whereby electrical cables embedded in a wall can cause an interference.
Hans.