SuzyM has a great suggestion. Basting or sewing with a long stitch first is so much easier to open if necessary, than constantly unpicking small stitches, and running the risk of distorting the block. CookyIN also has a great suggestion. Use your walking foot so that top and bottom feed at the same rate. I also use a bodkin (or my seam ripper if that is the only thing that is handy) and hold the fabric in place very close to the needle as I sew very very slowly.