Hi and welcome to Just answer.
That problem is usually cause of wrong tension. Try to adjust your upper tension and get your scrap materials and try it again. Adjust the tension until you get the right one, if you adjusted all the way but still the same. Try to clean your bobbin area and replace the needle.
If the upper tension does not response to your sewing or doesn't make any changes/reaction on it, you have problem on tension mechanism. You can also try to clean the tension disc, get scrap fabric and place it to tension disc and move it forward and back like you are cutting the woods.
Hope this information will be helpful. If you still have question just reply.
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Let me know,Chester