I don't think your problem is what the signal is going through, your problem is the direction of your antennas.
Omnidirectional antennas are not truley omidirectional. think of a ring around the middle of your antenna, and the singal spreading out from there horizontally.
Now, if you pionted your two antennas straight up, you might never get a signal if your computer is one floor up. Instead, make the two antennas parallel to each other, like this | |. What you have now might be something more like this --- ---. The second won't give you any signal, because the signal would be going parrallel to the reception, IE, they never cross..
Make your antennas parrallel, and then we'll go from there.
BTW, a good router and adapter should be able to get over 300 feet indoors, unless you have steel walls of course.