Another possible reason for the bowing is that you are ironing it on the board the long way. I read a site that said it is better to lay the strips crosswise to your ironing board and then press. If it is any consolation to you, the only bargello quilt I made, I had to sew twice because I didn't notice the bowing and my quarter inch seams weren't!! Once you get the culprit fixed the rest will go much easier. I speak from sad experience!! In the end it was a great quilt but I definitely learned about patience making it!