My experience with all sewing machines is that if either the upper needle or the bobbin is not threaded correctly, the machine will not sew period. This is what the shuttle bobbin looks like threaded in my treadle.
On this machine the needle must be threaded from left to right. When replacing the needle the flat part of the needle faces the right. Always test your stitches before sewing anything.
I do not have a manual. However, my Bernard Stoewer treadle is almost identical to the Singer 127, with the exceptions being the little hole on one side of the rod bobbin as well as Stoewer has a stitch length lever with a reverse, whereas the Singer has a screw to regulate stitch length and no reverse. The Singer 127/128 manual is available as a pdf for free from ISMACS (International Sewing Machine Collectors' Society). Page 8 and 9 of this manual provide excellent written instructions with illustrations on how to thread the bobbin shuttle. Hope this helps! Enjoy and have fun sewing.