1. Use the wrong sewing thread. If you do not understand the material or sewing requirements, the bottom line will not be properly matched.
2. The quality of sewing thread itself is flawed, such as: fuzziness, lack of strength, loose shares, lack of shares, etc.;
3, the needle number is not used, resulting in the size of the needle and the line does not match;
4. The needle or pinhole of the needle car is damaged or defective;
5, sewing thread tension is too large, machine sewing thread channel tension is not adjusted;
6. Incorrect threading or incorrect operation of the sewing machine;
7, pinhole fever, sewing thread does not have enough lubrication, that is not enough oil.
8, the feeding process is not smooth, or feeding method is not appropriate.