Some thoughts that come to mind.
Does the picture show the seams matching?
Go back and check the seam allowance on all your inner seams. If they were not exact then the long seams won't match.
If you have one petal that is not sewn, lay it down matching the end points of each cross seam. Make sure the original cut was accurate. If you put a pencil dot where the hortzontal and verticle seam meet at the ends and then pin at these places you should be able to know if the original cut was accurate.
How many petals in your Dahlia.