Put the antenna up higher

There are a couple of ways around it, some work better than others but LOS is certinly better is you can get it.

Best way to check is stand on either the roof of your home or work and take pictures with the digital camera. the decide from there.
Other factors also need to be taken into account like the area and how many other wifi/similar devices are runing that may impact the signal.

What area is it in?