Are you placing your squares on top of each other then marking a diagonal center line and sewing ¼" to either side of the line and then cutting on the marked line? If so, it may just be that you're coming in a little too close to the marked line and not the exact ¼". It doesn't take much for a block to be a little too large or small. When I make half-squares, I do sometimes have to square them up to size. I don't think I've ever had one come out just exactly right. It doesn't hurt to trim them up a bit. I hope this helps some.