The top thread is really loopy on the underside of the fabric, sometimes it misses a stitch, sometimes, the upper thread breaks, and it often gets so tangled underneath that the machine won't feed
Your tension is way too loose on top or too tight on the bobbin. Try adjusting the top tension one number at a time until you get it right. (Only adjust the bobbin tension as a last resort). Also you could have different thickness of thread in the bobbin than on the top or are sewing a very heavy fabric. Try adjusting the tension with same thread in both bobbin and top and use a normal weight fabric until you find the right tension to get you back to a normal stitch before going to a heavier fabric or thread.
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Your tension is way too loose on top or too tight on the bobbin. Try adjusting the top tension one number at a time until you get it right. (Only adjust the bobbin tension as a last resort). Also you could have different thickness of thread in the bobbin than on the top or are sewing a very heavy fabric. Try adjusting the tension with same thread in both bobbin and top and use a normal weight fabric until you find the right tension to get you back to a normal stitch before going to a heavier fabric or thread.