The 2 sections probably look flat, but are the 2 edges that you are going to join straight? The 2 edges have to be straight or you will get a cupped center. You could trim the 2 edges but then the center star is going to be off. You can also take all the sections apart and make sure that the center angle is correct on each segment. 8 segments = 45 degrees. You can either trim the sections or mark the seam line to get the perfect angle and sew on the line, taking a larger seam where the segments are bigger. By working with the individual segments, the excess fabric should be taken up across all of them resulting in a better looking center star.