In case anyone is wondering what we're talking about with the cotter pin, here are some pictures. First is the entire length of the rod from the feed dog drop knob to the connection to the feed dogs. Second is the rod in the "dogs down" position (no cotter pin); you can see a spring on the inside. Third is the rod in the "dogs up" position with the cotter pin in.
I think lowering the feed dogs compresses the spring against the cotter pin, providing some force to raise the feed dogs all the way when the knob is turned back to "up". Assuming this to be the intent I reinserted a cotter pin with the dogs up, spring not compressed.