Anytime a block calls for using a scant 1/4 inch seam allowance I over cut my pieces. How close to the 8 inch size is it? If it is too big then your seam allowance is too little, if it's undersize the seam allowance is too big. The bias edge on the triangles might be stretching but that should have helped with your problem. Measure the center block to see if the size at the seam line measures 5 and 7/8 across.