I'm not familiar with this machine but it sounds as though your top tension is too tight so that when you sew through heavy material the thread isnt slack enough to go through that extra thickness and so simply snaps. Try reducing the tension a little at a time and see how your stitching is affected. Another issue might be that you are trying to sew a heavy fabric and using too light a thread. You need to match your thread to the job in hand. For a thicker thread you need heavier top tension. If your machine has been correctly serviced you should not have to alter the bobbin tension at all under normal circumstances as thicker thread will automatically have a heavier tension as it goes under the bobbin case spring and so you have to match this heavier tension with a higher top tension in order to get a perfect lockstitch. I hope this helps.
Answers & Comments
I'm not familiar with this machine but it sounds as though your top tension is too tight so that when you sew through heavy material the thread isnt slack enough to go through that extra thickness and so simply snaps. Try reducing the tension a little at a time and see how your stitching is affected.
Another issue might be that you are trying to sew a heavy fabric and using too light a thread. You need to match your thread to the job in hand. For a thicker thread you need heavier top tension. If your machine has been correctly serviced you should not have to alter the bobbin tension at all under normal circumstances as thicker thread will automatically have a heavier tension as it goes under the bobbin case spring and so you have to match this heavier tension with a higher top tension in order to get a perfect lockstitch.
I hope this helps.