It is a little difficult to tell exactly what you say is wrong but, do you mean that there is just too much fabric at the corners?
If you are just taking a length of sashing and sewing it on then cutting it to length it will stretch out the side of the block and it will make the sashing do that little wing out at the corners.
Measure your block side-to-side through the center of the block, not the edges. Cut your first two sides of sashing the same measurement. Mark the middle (pinch a crease, use a pen, or a pin) of the block and the sashing strip. Match up the middle and the ends and ease in any fullness. It will likely seem that the sashing strip is too short unless your block is absolutely square since it tends to stretch on the outside edges.
Press the border seam towards the border. Measure across the block and the two borders and cut the other two sashing strips the same, sew them on the same way.
See if that solves the issue of getting them square.