When doing triangles, I try to use the method where you sew squares together and then cut. The bias edges of triangles tend to distort. With that being said, have you cut them all? If not try the other method of cutting squares, stacking 2 together, sew across the diagonal on each side of a line drawn from corner to corner. Draw a line from the other adjacent corner and sew on each side of it. You then cut along the lines to separate the triangles.
If they are already cut, starch them to reduce stretch and use lots of pins when you sew them together. Try not to stretch the bias edge too much and retrim them square if needed when you're done. Good luck.