Elastic thread won't give the full gathers you want. Did you use three rows of big stitches, secure one end by winding all three threads around a pin then pull gently from the other end - all three threads together. You pull the bobbin thread, not the top. As your gathers bunch up, hold the threads and slide the bunched gathers further toward the pinned end. When it's the length you want, wrap the threads around a pin and distribute the gathers evenly.