I know this may sound like an odd question but did you measure the pieces and cut them with a cutting mat or did you use a ruler? The measurements on the cutting mats are NOT CORRECT! If you measure them with your ruler, often you will find spots where it is off. For a couple years I just could not seem to get my blocks to come out the right size. It was frustrating and often made me want to cry. I just didn't join any type of exchanges or anything like that. Then I had a teacher tell me to measure my cutting mat and I was shocked to see that right in the center, where I did most of my measuring, it was off quite a bit. Once I quit using my cutting mat to measure with I was amazed at how much better my blocks came out. If you haven't tried this, measure your mat to see if you have one that is off. It could be that you aren't doing anything wrong and it's the mat to blame.