Check your seam allowances to see if they are wider than 1/4 inch. Make sure you are cutting the original sizes for the pattern. Check the ruler lines to make sure you aren't short cutting them.
What thread do you use for piecing? It can make a difference too. Finally, check the pressing of seams to make sure there isn't lost fabric in a fold along the way.
PS: I sometimes cutpieces a titch over the pattern size and then square up or trim as needed. Good luck!