This may not solve your problem, but--press seam allowances to one side. When putting together 2 blocks, have the seam allowances going in opposite directions (ex: blue piece to left, red piece to right) and pin through the stitching. If you're not sure, baste it first, then look at finished seam. They should line up just right. Make sure when stitching that the bottom seam allowances are not pushed in other direction, because this makes a bump and can throw off exact seam match.