is you bobbin horizontal or vertical. either way if you pull the thread watch which way it turns the bobbin, if it goes clockwise as in the hands of a clock, turn it over so the thread goes the other way anti clockwise and give that a try, some machines like mine goes better in the clockwise position, others in the anti clockwise position. it is all trial and error, also check that the top thread runs smoothly as this can also affect the tension.