Sounds like the screw is stripped. A visit to repair many less you know how to take the screw fixture off and replaced. I worked on a friend's singer the other week which was missing stitches and not sewing correctly. After altering to correct place all the tensions I went to take the needle out to replace. The gentleman who had borrowed the machine had jammed the needle somewhere, definitely not the right place. After removing needle and putting a new one in correct place the machine purred.