Your problem may be this:
If your block strips are cut on the cross grain of the fabric - from selvage to selvage - then the cut ends, where you sectioned them for the sashing, are on the straight grain....with no stretch or give at all.
Then if you cut the sashing strips on the cross grain of the fabric - from selvage to selvage - which has a bit of stretch or give, you may find your pressure foot is pushing the sashing strip along ahead of the 'toe', causing this misalignment.
To solve this, always place the strips on top of the sashing as you are sewing them together.
Sometimes we just plain forget that grain in quilting really can affect the finished product, sometimes! I know I do!
Jan in VA