Hello Nikki,
First of all make sure the top tensions is set between 4 and 6 and make sure that when you thread the machine the presser foot is up, make sure you can pull the bottom tension through with some slight resistence, if it is hard to pull through you need to slacken the large screw about a quarter of a turn on the bobbin case, make sure you use a good mercerised thread, Singer machines are delicate when it comes to the thread being used.
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