There is a trick to butting up seams going opposite directions. If you have the top seam allowance going away from you and the bottom seam allowance coming towards you it should work every time - assuming you were careful in feeling for the "butt". When the machine is feeding fabric through the feed dogs - they take the bottom fabric and "pull it" toward the back of the throat plate. So if you have your seams the way I just said, the machine will help pull the bottom seam allowance toward the top seam allowance for the "butt". This is a very simplified answer and I'm sure there are lots of other tips - but this one works for me. I haven't had problems with seams matching since I started doing it this way.