I think the problem is not actually with the sashing but with matching the blocks in the adjoining block. I can see the problem but only if I look very carefully. If this bothers you then rip out and resew matching the adjoining blocks instead of the sashing. This may mean that you need to ease the block into the sashing due to block being stretched. If it too stretched to ease easily then dampen the block by spraying water and allowing to dry. Fabric will usually shrink when damp and may fit better. Also put too large side on the bottom when sewing to help make fit. Good Luck!!!!!!!