I just finished a full-size “Nana’s Garden.” I used good 100% cotton fabric. My middle squares are about 5 1/2 inches. I did not quilt in them, used 100% cotton batting and stitched in the ditch in all seams.
Now that the quilt is in use, the middle squares get wrinkled all the time, yet nothing else wrinkles. I can smooth the wrinkles out, but wonder why the middle squares are the only areas in the whole quilt that wrinkle.
I suspect you’ve run into a basting problem. More accurately, a “lack of basting” problem. Did you baste? Either with thread, pins, spray, gun, or fusible batting?
It might be that the centers of those large blocks got twisted or misaligned before you got a chance to quilt them. Try basting more next time.
If you basted well, it might be a quilting problem. If you quilted by hand in a hoop, the top layer of the quilt may have been stretched more than the backing.
This is a common problem with hoops, especially if there is not a lot of basting to hold things together. The action of placing the top hoop over the quilt and the bottom hoop will naturally stretch the top unless you open the hoop as far as possible. Few people actually do this, although they should.
Instead, they unscrew the join a little bit, just enough to wrestle the quilt in, tighten it back up, and then pull at the backing to get the tension between top and backing the same. Guess what that does to the un-stitched, un-basted layers?
If you quilted by machine, my guess is that you may have a tension problem. Most machines feed the bottom layer faster than the top. If the quilt isn’t (you guessed it) basted well, then all three layers of the fabric sandwich could move under the needle at different speeds.
If you are machine quilting, it would also help to have an even-feed, or walking, foot to help all the layers of the quilt pass under the needle at the same speed.
There is not much you can do about it now, unless you want to do some major ripping. Or, there is a chance you could fix the problem with more quilting (in the centers of those large blocks). But perhaps you liked this particular pattern because there wasn’t a lot of quilting!
In any case, baste more, drink lots of water, get plenty of rest and exercise, and love the quilt as it is.