Go back to the beginning. From studying your photo, the piecing is not accurate: i.e., white point is cut off, seams not meeting. I never use starch. Just wash the fabric. I have a feeling that the fabric is being stretched during all that starch pressing. And if you have a 12-inch block, the sashing must be 12-inches also, plus seams, of course. To add sash, I've seen some quilters just start at one end and stitch to the end of the strip. This doesn't work as the blocks can be different sizes or the edges stretched. If a cut size block is supposed to be a 12-inch finished size, but the edges measure 12 3/4 inches, the extra length (1/4 inch) must be worked into the top in the location where it belongs. If it is 1/2 inch too long, the same thing applies--work in the excess where it belongs.
Best of luck. SandyQuilter