If the single stitch does double stitches there can be two ways to fix it:
1. If the quilt you are working on is lighter than the one on which the machine was set up, the needle will be too fast for that material. You can tell this by the needle moving fast during single stitch and the take-up lever overruning (stopping beyond its specified "UP" position).
In this case the needle positioner speed needs to be decreased. Please refer to the NEEDLE POSITIONER SPEED section of the adjustment instructions and do adjustments with the LASER light off (DOWN light on on Nolting machines). Watch the take-up lever stopping lower and lower as you do the adjustments, and stop the adjustments when the take-up lever stops at or a bit before its "UP" position.
2. If case #1 does not apply, the needle down position needs to be moved BACKWARD. Please refer to the UP/DOWN POSITION ADJUSTMENT section of the adjustment instructions and do two-three adjustments of the down position with the LASER light off (DOWN light on on Nolting machines).