Antenna needs a clear view of the sky in order to function.
To provide you with an example, I have my Sirius antenna mounted on the back window of my car. When I drive into my garage so that the antenna is on the outside, with the garage door open, I don't always get reception.
However, it could possibly be due to only having one direction from which to receive a signal in that case. I've been inside a large stadium for car shows before with my car, and I always get Sirius reception in there, even when I am in the airlock. I couldn't tell you why it works in there and not in my garage.