Difficulty which stitching can be caused be incorrect thread tension. To correct a thread tension problem first identify how the thread is breaking. If the thread is breaking, it may because the top tension is to tight and needs to be loosened. Alternatively if the thread is looping in the back, the problems is most likely top thread tension which is to loose and needs to be tightened.
If you are having difficulty with stitching decorative stitches and have already adjusted top thread tension in everyway imaginable, the problem may lie in the tension of the bobbin tension setting. The tension of the bobbin must be loosened to get the desired stitch tension. In the past it was adviced that the bobbin tension setting was not to be touched, but that is archaic advice as it was given before the machines stitch types and amounts were as vast as they are today.