We've talked about line of sight before.
Somehow, it seems like a hard concept to imagine, so let's take it really simple. Point yourself at something (like the sun.) Is there anything between you and the sun? A tree, another building perhaps? If you can see the sun (and for gosh sakes don't stare at it) you have a clear line of sight to the sun. If you can kind of see the sun, you have an obstructed
line of sight to the sun. If you can't see the sun, you have no line of sight.
Line of sight is critical when aiming satellite dishes and antennas. With satellite dishes, only a clear line of sight will do because the satellites are so far away. With antennas, you can sometimes get away with an obstructed line of sight because the signal is stronger.
Anything you put in front of a signal is going to cut the power of the signal. Even air cuts the power of the signal. If you put an antenna inside, its efficiency is cut by about half.
What can you do about line of sight issues? The best answer is generally to go up,up, up. The higher you go, the more likely that you'll find a clear line of sight to whatever you're looking for.