Are you doing your blocks in flannel? Flannel is harder to get precise piecing and the seam intersections will be bulkier then quilting cotton. A seam guide on your machine may help with accurate piecing. You should also check your seam allowances to see if they are 1/4 inch……however…if you are sewing with flannel , I usually do a larger seam allowance because the weave is usually looser and they tend to fray.