The stretchy seams happen when the bias is on top and drags against the foot. Two bias pieces are difficult so match a bias with a straight of grain.
Plan the entire star with a straight grain edge next to a bias edge. Cut your diamonds so that two sides are on the straight of grain. When you sew two diamonds together make sure to have a bias edge and a straight edge. Put the bias edge down next to the feed dogs.
FYI: this technique can be used when sewing a longer piece to one that is a bit shorter. Also known as 'easing'.