I'm not too sure on 'correct' terminology - but will explain best I can. Some thread is caught under the bobbin case thingy and machine won't sew. I was doing zigzag stitching and clunk. Needle was jammed in the bobby case thingy. Removed needle, cut threads best I could. I've tried getting the thread with tweezers but no luck. Handwheel won't turn. Hope I've explained.

make sure the bobbin is counter clockwise

make sure top thread has not come off thread take up lever

could also be a timing problem

try cleaning cleaning the tension disks, remember you tension setting,, take some unwaxed dental floss and go between the disc's to clean in them, and then reset the tension disc's by to normal

If the bobbin is inserted with the thread spinning the wrong direction this will cause "birdnesting" - thread build up under needle plate, you need to cut thread to get the item off machine. flip the bobbin over into the case and try again. Also make sure you remove ALL thread caught up in "Birdsnest" when they happen.

Cheers,
Kelly
Embroidery Services
San Diego, CA

Try these suggestions
1. Change the needle--use a needle that's for the fabric being sewed
2. Rethread the machine--top --make sure to use every thread guide
3. try a different bobbin--make sure bobbin is inserted correctly into the bobbin case, & make sure the bobbin is turned the correct way in the bobbin case
4. make sure the thread has not come off the thread take up lever
5. Clean under the needle plate & in the bobbin area for lint or threads

Check for thread caught under silver guide, may have got jammed in there.

I had the same problem and tried everything you did: rewinding the bobbin, checking to be sure I had the bobbin in correctly, checking the upper threading, readjusting the tension. When nothing I tried helped, I bit the bullet and took it to the shop where I had bought it. The timing was off; there were burrs on the bobbin case; the hook was not performing correctly;there were threads caught between the tension disks. It cost about $65 but when I brought it home, it purred like a kitten! I suggest you take it in for a check up! Especially if you've tried all the usual tests and none of them worked.
I'm sorry I couldn't give you a do-it-yourself answer but sometimes they've just gone beyond what we can do for them.

the needle might be bent-roll needle on flat surface & see if it rolls correctly
if the needle is make a popping sound when it goes thur fabric the needle is blunt
check the bobbin area to see if there is lint or loose thread
check to see if the bobbin has become warped
the bobbin may not be inserted correctly in the bobbin case
check to make sure the bobbin case is inserted correctly into the machine
rethread the sewing machine in case the thread is caught somewhere
make sure there is a thread tail of 3-4 inches on both upper thead & lower thread

hello, I had the exact same problem and could not find the solution until i took my machine to my sewing sensei and i solved my tedious problem. It was actually not a tension problem at all but a threading problem. Make sure you have threaded your bobbin correctly and that when you pull on the bobbin thread that the bobbin turns the right way. (mine must spin anti clockwise to work)  Make sure that when you pull up the bobbin thread using the needle that it is not over the feed dogs connective bar but under it. A big problem when the thread gets caught around the bobbin case wheel is that the thread is looped around the feed dogs once and that causes the thread to get caught around the bobbin wheel. In conclusion, thread the bobbin correctly and read over your instruction book before spending the money to get it fixe in s shop.