First you need to check the correctness of filling the upper and lower thread. It would be better perenapravit scheme, which is specified in the operating instructions or marked on the case of your sewing machine. If the thread is seasoned correctly, no sewing machine stitching will not. Always make sure that the thread went from the spool freely and never cling and do not get tangled up.
Check the regulator tension of the upper thread.
Need to replace the needle because it is the most vulnerable point of the sewing machine. She often suffers from improper filling or improper use of the machine (stretching of the material when sewing, the discrepancy of a needle stitched material, etc.). Be sure to place the new needle and discard the old. In any case, do not use damaged needles.
Pay attention to the quality of thread with which you sew. Check them for strength. A good thread needs to be strong enough. Look at them: the threads should be uniform in thickness and they should not be nodules. Note the thickness of the thread, they may not be suited for your sewing machine. As a rule, recommendations on the use of threads to write in the instructions.
Clean machine. Often, the cause of failure is contamination of the Shuttle device and engine tissue. After the readings put oil all moving joints.