Go back to basics. Check that the needle is correctly seated, or better still fit a new one. Make sure you have the same weight thread in the bobbin as on top. Remove, clean and retread the bobbin. Retread the upper thread ensuring that you follow the correct path and pass between the discs of the tension unit. When rethreaded, gently pull the upper thread through the needle. You should feel a slight resistance and should see a slight deflection (bending) of the needle, just 1 or 2mm at the tip. No deflection? Increase top tension. Excessive deflection, reduce tension. Finally put some fabric under the needle and lower the presser foot. Make sure you can pull the fabric towards the back of the machine but not towards the front. Increase or decrease presser foot pressure as necessary. Make sure the needle is not in the fabric for this test. Hopefully this will cure your problem.
Report back if you still have difficulty. Good luck.