Mo-rie,
This might not be of any help, but since I have had to do this on some of my machines here is my suggestion.
Set both your 301s side by side with the left ends facing you. Take both face plates off. Then compare the one that has the problem with the one that works right. Something is different and if you study them both it will show itself. The problem might not be in the needle bar area, but hopefully it is.
Joe