Lots of helpful hints here. I don't know if it is a particular block you are doing but I assume that is the case. Another factor I didn't see mentioned is fabric construction. Some weaves will not allow you to make a point where you want to! They have a propensity to shift in one direction or another. A friend tried to do a Hunter Star with a twill fabric that you could NOT sew straight! Didn't matter which direction the fabric was cut - it would not work! Is your fabric evenly woven? Try the pattern with other fabrics... and good luck!