Here's the video Watson is talking about, for those who want to look.
https://www.youtube.com/watch?v=lwuv4oYXOqY
I agree with Onebyone. You have to ease the fabric in. In the video, if you watch her hands carefully, she's easing the fabric in the middle. She gets one corner set on the tape, then the other corner set, then she moves back to the center and sort of massages the piece that is too short so it stretches a bit to fit. Once you grasp how to ease in fullness and do a few practice blocks, you likely won't need the tape anymore.