I don’t know if I’m just an incredible novice and this isn’t the same problem, but I was having this issue and couldn’t figure it out, then came to realize that when changing stitch lengths I needed to manually lift the reverser arm all the way up then use as normal. When I used the dial to adjust stitch length, it does move the arm up some, but not as far as it needs to be, which seems to result in short backstitches. Just throwing it out there as a possible solution. I know this post is a few years old, but being as it’s one of the first google results I got, thought it might be worth posting.
