Which phase is it in? If its a waning gibbous phase, and the Moon is near the horizon after sunset you need to draw the line the other direction, from the Moon to its closest horizon, beneath the Earth, to the Sun, rather than across the sky to the western horizon. Just guessing, but I suspect you're not drawing the shortest line possible.

I live in San Francisco, at latitude 38, but even when I was in Antarctica I noticed the Moon's terminator was perpendicular to the Moon / Sun line.

See if you can take a picture to post here next time you notice the terminator pointing the wrong direction. Try to zoom out as much as possible, and include horizon in the shot. It should be pretty easy to figure it out from that.

Here's a picture I took in Antarctica. The Sun is not in the picture, but it is exactly where you'd expect to find it based on the Moon's terminator. It is about 90 degrees to my left, and slightly higher than the Moon.

Click here for the bigger version:

http://orbitsimulator.com/Antarctica...G_71891024.jpg
http://orbitsimulator.com/Antarctica...G_72191024.jpg