I'm not a hand quilter so I'm just going to guess.
My first question is how did you baste it? Pins? Thread? Spray? And was it like this before you put it in the hoop?
If there was nothing wrong with the basting method, how tight did you have the quilt in the hoop when you quilted the center embroidered designs? Wondering if it was too tight, perhaps you stretched it.
It looks like there is only a small amount of quilting (corners only?) on just the embroidered squares. If so, I'd remove the quilting that you have in there already, re-sandwich it and if you didn't use enough pins/thread/spray make sure you do this time, then hoop it loosely before you quilt it. you might also consider quilting the center piece, then the sashing around the center, and work your way out - instead of leaving all the sashing until your done.
It's a shame. Very pretty quilt.