Did you do it on a very sunny day? The window in my bedroom faces the sun from midday through the afternoon; and I was able to charge my battery all the way. Just make sure the watch face is always in the sun; and whenever the shadow from the side of the window was creeping closer to the watch, I would move the watch so it was in the middle of the sunlight again.
How long do you leave it to charge? Like I said, make sure that it is in complete sunlight, and leave it there for a long time, as long as there is direct light from the sun to charge it up. I basically left mine on the window nearly all day. You will be without your G-shock for that day, but when you get a full battery charge it is worth it.