It almost appears that the white sections just above the "off" bands are different sizes. Are you extremely accurate with sewing on the line? Make sure you do not stretch the fabric at all. Sometimes it helps to pin the points of the patches where they meet. I put a pin straight down on one point and match it to the corresponding point on the next patch. (One pin through two pieces - do not "weave" the pin it). I don't know how to draw this, but I will try to use my keyboard to illustrate:
l
l pin
l________
l
l
l_________
l
Hope this is clear. After I match the points, I "weave" a pin through the patch to hold the seams accurately. Remove the up/down pin and sew.
Also: how are you copying the foundation papers? Make sure you copy all of them at the same time, on the same printer/copier. There are slight variations between copiers and this will throw off your accurate seams. To make sure all foundations are the same, hold a couple (one on top of the other) up to a strong light and see if they match up.
If you need to unsew a seam, rip out the stitches, then use scotch tape over the printed line (Scotch makes a removable tape in a blue plaid dispenser. This works really well.) Resew though the taped line.
Can't think of anything else at this point. I do a lot of paper piecing and sometimes I, too, have these problems. Often times, you can fudge the seams - a little stretch here, a resewn line there.
Good luck. Hopefully others here on QB will have more suggestions.