Scratched bobbin case

SOURCE: Sewing Machine CS-8060

I do have the very same problem! I took my bobbin case out and took a good look at it, turns out right below the plastic hook part, the plastic was cracked off (probably due to thread jamming). Turns out I need a new internal bobbin case, which range in price from $35-$50 from what I researched. I had no luck with local sewing centers on finding the part.
That may (or not) be your issue as well. Good luck, and I hope this your issue is simple fix. A little tune up after might be needed, HAPPY SEWING!! ~Kelly J~

SOURCE: Bobbin Case

ok Sim this is a bit tricky but not to tricky first turn the handwheel toward you until the hook is at one o clock postion then slide the case without the bobbin in into the hook area you may have to work the handwheel back and forth alittle but it should slide in just remember that the case should ride in the rim. also take the needle out first that way you wont get stuck trust me it happiens.

SOURCE: thread knotting around bobbin case

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.