Turning reasonably smooth is enough for the tensioner, as long as it turns from about 0 at the outer limit and closer to 9 on the most inner setting. They usually don't go spot on 0 to 9, nearly so is as good as it gets. If they are wrongly adjusted they will stop half way, turn from 0 to 5, or 5 to 9, and not a full turn. I don't think this is the problem with the revers stitch.
Do you mean no locking or not loops on reverse stitching at the moment? Double check you have the maual for 15-91. It treads from right to left, and the needle goes in with the flat side to the left. Model 15 comes in various sub models and minor differences, this is one of the more significant ones. Some of them are the other way around.
Last edited by Mickey2; 10-21-2017 at 09:34 AM.