Sunnye,
The 27-28, 127-128 are VS (vibrating shuttle) machines. If you got round bobbin instructions you didn't put in the correct model number.
As for the instructions, sometimes they are too simple for us modern folks who are used to very complex things. Remember back in the old days education was reading, riting and rithmatic. Machinery was simple. So channel your GGGrandmom and it will come to you.
Joe