1. Make sure the needle is fully seated and tightened down.
If that doesn't solve the problem.
2. Check that needle is correct size.
If that doesn't solve the problem...
3. remove the foot, and remove the plates in the bobbin area. Get a magnifying glass out and inspect the shuttle race/bobbin carrier. Can you see the stab marks? Rotate the handwheel toward you slowly until you see the needle hit. Is the hook behind the needle? If so your Timing is Advanced and the machine needs to be timed. Have it done professionally. It is likely that you may need a new part or two as well.
Answers & Comments
1. Make sure the needle is fully seated and tightened down.
If that doesn't solve the problem.
2. Check that needle is correct size.
If that doesn't solve the problem...
3. remove the foot, and remove the plates in the bobbin area. Get a magnifying glass out and inspect the shuttle race/bobbin carrier. Can you see the stab marks? Rotate the handwheel toward you slowly until you see the needle hit. Is the hook behind the needle? If so your Timing is Advanced and the machine needs to be timed. Have it done professionally. It is likely that you may need a new part or two as well.