She said button hole stitch, not blanket stitch....not sure now what she is using...if it is button hole, why would you use invisible thread for buttonhole stitch? Try using 90/14 topstitch, since you are going thru so many layers. BEAUTIFUL WORK.

Lady Diana - oops! You are so right, I totally misread the OP.....now, I too wonder why such a heavy stitch around the center...a little zigzag with invisible would work perfectly....JMHO