When I first started doing these 3 yrs ago mine were smaller as well. I spoke to my teacher about it. One of the little tips she gave me really suprised me that worked; I switched to Aurifil thread. It's 2 ply. You wouldn't think that wouldn't make a difference but it does. My first blocks were suppose to be 6in and when you have all of those seems going across the block. I also square each section as I go. She's great as she lets us know what each section should be so that we can so that as we assemble the block. I also use less than 1/4 in seems because I need that extra allownance. I love the quilt but even after 3 yrs I still have trouble getting them to come out to size. You could just make them all 18 instead of 18.5