After reading through all the suggestions, I think I know what caused it.
I always measure through the middle, lengthwise and widthwise, before cutting my border. Did that, so this isn't the reason.
In the list of suggestions that was given, I'd done everything correctly, except for one. I did cut the border WOF. I've done this many times before with no problem, but I suspect those fabrics weren't as stretchable as this one. I now think that the WOF was not a good thing for this material.
I'm going to try the gathering first, if not pleased, I'm trying the darts. If still not good, I'll take it off again and redo. My lazy-man's method may make me do more work in the end. I probably should do the frog stitch (rippit, rippit) and start over. LOL
Thank you for all your suggestions. It's certainly been a learning experience and I've learned again from this failure through all your 'reasons why' this happened.
Many thanks and if successful, I'll post a pic to let you see solution.
Dray