Try sewing your seams a scant quarter inch, Sometimes if you sew the full quarter inch, you can lose some in the pressing step. After you have sewn two strips together, check your width of them making allowance for the seam allowance of course on the unsewn edges. I would guess it is in your sewing of the seams, but without seeing them of course can't be absolutely sure. Best bet for now would be to cut them down to the width of your blocks. Hope this makes sense. Been there and done that type of thing, so don't feel you are alone.