the problem is the pinning...you have two choices... match the ends, sew two or three stitches and stop with needle down, now nestle the seams together and hold them till they go under the foot, then match the second set of edges and go... the other method is to put one pin straight into the seams thru both pieces, right on the 1/4" mark, with the seams going the direction you want and then....hold the pin straight UP and DOWN while you place a pin to the left and another to the right...the center pin MUST stand straight up during the whole process. Now remove center pin and sew. When you pin just ONE pin into the center and then fold the squares to take the second bite, the squares move just enough to UN-match them... eventually, you will probably adopt the 'no pins' technique but the three pins method works well...