I agree on the alternate sewing. It is not anything you are doing wrong. This is a very common problem (the bowing of strips) in strip peicing. By switching directions of sewing every other strip you even out the bow somewhat. Many bargello patterns include this step in the directions.
Edited to add this link
http://craftnectar.com/2011/06/30/ev...sewing-strips/
See, it is NOT just you!
Last edited by feline fanatic; 06-20-2012 at 09:35 AM.