Check your seam allowance for evenness. I bet that the amount of the seam allowance varies. If so, that is your problem for having trouble making things match. Be accurate when you cut and stitch to get really accurate finished patches. Also, when hand piecing, don't stitch into the seam allowance and you'll have more "play room" to manipulate the seams to meet. Good luck--you chose a simple, but challenging size.
SandyQuilter