Some of this comes from the direction of the grain (the stretch). You have to sure not to pull on the fabric. Just keep it flat.
When sewing on the center piece...I started with a pin in the center of the cirlce piece then matched it to a center seam on the blaze and pinned. Then the two sides; then center of each of those, then center of each of those.
When stitching...be sure fabric is laying flat between the pins and hold before sewing. If it not flat, have needle down and pull slightly, just enough to flaten it out.
Hope this helps.
And...when you take the paper off the blocks you already have sewn, you may find that they will be OK (again because of the grain and stretch). Sometimes it works at one time and somethimes at other.