Originally Posted by DogHouseMom

I used to have this problem a lot (and I was sewing a LOT of 1" strips) and then I got a different machine that has a built in walking foot. I started using the walking foot (with the 1/4" guide) to piece and I will never piece without it again!!

I suspect that your fabric is not feeding evenly, not by a lot, but enough to create the bow. Since I started using the walking foot I haven't seen a bow.

Yes, you can finger press it to get the bow out, then press the seam flat (before you open it). When you are ready to press the seam open be very careful to make sure you keep it straight. I know your supposed to iron open seams from the middle and work your way out ... but when I have a bowed seam it's easier for me to finger crease the seam open, hold one end of the fabric with my left hand (hold it flat against the board), and press open starting at the right side and "walking" the iron toward my left hand. You have to make sure that you don't twist or pull the fabric and take it off the straight line while ironing.

If you don't have a walking foot ... try pinning the strips. And when you pin put one pin in perpendicular to the seam and the next pin horizontal to the seam line (but out far enough so it doesn't interfere with your foot). I learned this trick from Sally Collins, but after using my walking foot I rarely pin anymore.