Thread Gets Bunching up down the Fabric in My Sewing Machine

It does sound like it might be a tension problem. When you adjust the tension too much it gets off. It happened to my machine and I had to take it in to a repair shop. They did a complete cleaning and adjusted the tension. good luck!

The needle, per the type of fabric you are sewing, may not be compatible.

You already tried the tension.

But the upper tension and the lower tension has to be the same. If not, it is incongruous.

Also, make sure, the thread on the machine, IS threaded properly... thread it through the machine again.

Make sure the thread on the bobbin, is also wound, correctly, not loose.

Of maybe change needles on the machine. Maybe it is dull.

It could be a couple of things:
Does the machine need cleaning/oiling?
How is the bobbin tension?
Is the timing off? Did you hit a needle while sewing? You might consider taking it in be cleaned/tuned. That usually makes all the difference in the world.
Having said that, my first machine was a singer. It did the same thing you are describing. It wouldn't work even after all the above. I wanted to throw it out the window and give up sewing altogether. I hear that is common with singers. I now have a pfaff, and it sews like a dream. I love sewing again.
I hope yours works well after the above are assessed.

If it's not the tension try replacing the needle. I had this happen once and it drove me mad that I couldn't figure it out. I checked the needle and sure enough it was dull. I replaced it and didn't have anymore problems.

The bobbin might be too tight.

.

Are you using the correct needle for the fabric you are sewing?

There is upper tension and lower tension. You may need to refer to your manual to adjust both the upper and lower tension correctly.

Take a different empty bobbin and fill it then try it instead of the other one. Also re-thread the machine, make sure you are doing it exactly correct. If it is still doing the knotting up underneath you may need to take the bobbin holder out and blow out any debris, or use spray air in to clean it. If the bobbin holder is loose it may need to go to the repair shop to be tightened. Mine had that happen recently.

There may also be a small screw head somewhere down there that is the tension for the bobbin. Mine is on the actual metal holder the bobbin fits into. Some have a holder that comes out and then is placed back in with the bobbin thread woven through a tension device.

I also have a machine where the bobbin just drops in a round hole then is covered up by a metal piece. Then the thread is woven through a device on the permanent part of the machine.

You may also take the machine back to the store where you bought it and ask them to check it for you and do some simple adjustments while checking it out.

Could be something as simple as holding both the top and bottom threads when you start out. If you don't, sometimes the lower thread gets pulled back into the machine causing a knot. Also check to make sure your presser foot is all the way down, that can cause it also.

Sounds like the bobbin isn't inserted in the machine correctly.