Hi Carolyn;
Understand when the hook picks the needle thread up it has to carry it around the bobbin. There must therefore be enough clearance between the bobbin case & the holder for the thread to pass through. I have never seen the specs for the clearance of a W&W machine. However specs for a Singer VS machine call for 015" to 018" clearance between the shuttle & its race for this purpose. The difference in thickness between the #9 & D-9 bobbins which I measured was about 027". If the bobbin drop, or holder, is properly adjusted for the #9 bobbin there will be no clearance at all with the thicker D-9 bobbin, so the holder has to be backed up until the thread will pass around it without binding.
One suggestion I might make is, ordinary 20 pound printer/copier paper measures a pretty uniform 004" thick. Thus 4 layers - 016" which is in the tolerance range for the Singer. Cut a strip of this paper of a width that will fit in the holder & fold it twice lengthwise giving the 4 layers. When you try the thicker D-9 bobbin back the holder up a bit & lay this folded strip of paper between the holder ring & bobbin case, fold it up & then push the holder up until the paper is snug. Tighten the holder at that point & it should sew well. You could try the #9 bobbin at this same setting but suspect you will find it rather loose & rattly. Should it work well you could readjust the holder for the thicker bobbin, but again I highly suspect you will need the different setting for each.
Glad you found the bobbin photos helpful. I also did a write up on them in the files section, if you haven't looked at it you will find it on the 2nd page.
Miller/TN
