Just my thought if I understand the question- if you are sewing all the "twosies" into "foursies" from the same direction, (bottom to top, or top to bottom) you do get kind of a curvature that makes your strips uneven. Try sewing every other seam set from the top, then the bottom to keep everything from growing strange? As you join the rows, finger press a crease in each tumbler to match them perfectly and pin at those creased areas instead of at the seams since you can't do that. Might straighten it all out for you. I hope this make sense to you. It can also help to be sure all tumblers are cut from the same direction on the layer cake. Keep the stretchiest side on the east/west on the tumbler, so the top to bottom does not stretch. It really will help. Give the pieces a fabric a little tug, if it 'grows' that will be the side to side direction, and will not misshape your rows.
Last edited by madamekelly; 11-30-2016 at 03:37 AM.