Have you checked the needle size? Is it the right one for your heavy duty thread? Is the bobbin thread spooling off the bobbin case in the right direction? Test this by taking the bobbin out, turn it around and put it back in. If there is no change then that's probably not the problem. Have you tried adjusting the top tension? I suspect the top thread may be too loose. If you're not comfortable messing with the tension (it takes some time to get it right) take it to someone who is familiar with your machine and can do it for you.

Oh yes! Is the needle inserted properly? The groove faces out. This groove helps to guide your thread into the eye so would not work if in backwards.

Are you using the correct bobbin for this machine? Or is your bobbin damaged? Does the problem persist if you use a different bobbin?

If it's not the bobbin, it sounds like the bobbin case may be coming unseated. This can happen if there is lint or stray thread fragments caught underneath. Can you remove it to clean around it? If not, a professional service may be in order.

One other suggestion is to check the type of thread you are using. A sewing machine repairman once told me that one thing that will make thread bunch up is the budget type thread. The reason is it is not as smooth and even as the better threads and this makes for a lot of lint buildup and the machine always trying to deal with the change in the tension. Maybe check to see if the thread you are using is not a well known brand, clean out the lint and retry sewing with a better thread.

If your bobbin thread is being taken under the feed dogs , you need to pull out the bobbin thread and the top thread out a ways and hold on to both tails of thread while you slowly take 2 or 3 stitches by turning the wheel once you have those stitches down, then you can release the tails. Hope this helps.