It needs 3 or more satellites to "triangulate" on. My guess is that there aren't 3 satellites within the narrow field of view of a window.

 

Mine can get fixes indoors, but it depends on the particular indoors...doesn't always work. I've had other phones and Garmin-type units that have been unable to get GPS fixes indoors, or even outdoors in downtown San Francisco in between a bunch of skyscrapers...that's just how it is.

 

I've had mine tell me I was in the Atlantic off the coast of NC. I was driving in Ohio.... It's a little flakey some days, but most of the time it's pretty good. Inside you're going to get mixed results. Some buildings are more radio transparent than others.

 

check out GPS Test to get awesome real time GPS data - # satellites, strength, location in sky, etc.

see what it reports while inside.