Are you sewing the whole length of the fabric, 40" or did you cut them in half and sew 20". That is one important thing that Mary Sue Collins talks about in sewing small strips of fabric. Also, if there is still a problem at 20" decrease the length one more time. Less chance of the above comments to occur. I have done several of these strips, I love bargellos, and this can happen with larger pieces as well because something just was not cut correct to begin with. Another thought might be the top pressure coming down on the presser foot to the fabric, you may want to either increase the pressure or decrease. Did you starch your fabric before cutting? This would eliminate any stretching that might be happening. If you ahve cut them, then go ahead and give them a starching and ONLY PRESS!!!!! no ironing. That should allow some stability in your fabric and will match up with that batik you are working with. let us know how it goes again.

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.

I agree with the walking foot suggestion, though I would pin even when using it, especially because you are sewing a batik to a regular cotton and the thread count is very different from one to the other. My guess is that they are not feeding evenly for that reason.

Did you try starching your fabric before cutting the strips? I starch mine till it's almost cardboard stiffing and the cuts are perfect; also, the starch prevents the fabric from stretching when sewing strips together.

Machines that do not have dual feed - the bottom layer next to the feed dogs frequently feed in that layer a bit faster/more - that's why 'in the olden days' the layer that was to be eased was on the bottom.

1" strips bowing
 

I have found when cutting strips WOF that I sometimes have to stop and measure to see that I'm cutting straight from the fabric and even it up before I start again. It seems along the fold it will get off a bit.

Strips cut from length of fabric stretch less. DH and I have started cutting 20" of fabric or whatever works for the pattern you are using and cutting our strips from the lengthwise grain of the fabric instead of the width of fabric

don't forget - when pinning on batiks, the hole of the pin might remain. i would suggest pinning on or within the seam allowance.

it's also important to switch directions when sewing strips. If you sewed the first to strips down one direction then on the next strip sew up direction.

I agree with Silver Needle and try to always cut strips the length of the fabric and even then, I take steps to cut on the straight of the grain. Just because it's laying straight on the cutting board, doesn't mean that the threads are lined up straight. I have had smiley strips particularly when I cut a fold and the threads weren't straight even if the edges of the fabric were.