It could be something like a piece of thread caught in the uptake arm on top. Unthread the machine and try to thread all over. If it is tension on the bottom then the bobbin is OK. It is coming from the top.

Sounds like your upper thread is not in the tension disc or is out of the take up bar.

Typically, nest on the bottom means the upper thread is the problem. I had my long arm doing what your machine is doing and finally, found out the thread on the top was catching on the notch that you use to keep the thread from unwinding after using it. It would sew okay a little bit, then mess up royally. I started watching the thread all the way through and sew it when it happened. I repositioned the thread spool and no more problems.

Another time it happened, it was a tiny bit of thread in the thread path up behind where I wasn't supposed to take my machine apart. The tech found it for $125. I know how to take that piece off now.

Another time, I tried using an L bobbin when I was supposed to be using a Class 15. The top thread kept catching on some metal surrounding the bobbin case. This time it was top and bottom. Changed the bobbin and everything was fine. I have marked several bobbins that are Class 15 to use for comparison. Now I wonder why the bobbins don't come with a notation as to what number/type bobbin they are? Now if I start having problems, I check bobbin height first. My L bobbins are about 1/32nd of an inch shorter than the Class 15's. Not much, but enough to cause problems. I am actually in the process of checking all my bobbin heights and moving the L's into a safe place. The thread on them is a very tiny polyester that I will use as a top thread to stitch around applique.

Another time, I didn't have the thread seated properly in the upper tension discs. It looked okay, but was just sitting in the discs....someone else had threaded the machine. They didn't tell me it was messing up on them. Re-threading stopped that problem.

With the long arm, I had some bobbins, that weren't wound properly. From the mess under the bottom, I thought it was my upper thread...nope. I looked at the bobbins carefully and realized they weren't wound evenly. I pulled off half the bobbin thread and tried again. Worked perfectly.

If somehow or other, your machine has jumped time, it will do the same thing. It's more likely to break needles and such, but if only out a tiny bit, it can make bird's nests. Now, that requires a professional for me.

Good luck!

Were you able to resolve it? I would love to know how you fixed it too.

So I walked away for a while. I just came back and changed both top and bottom threads to a thinner thread and this seems to have stopped the issue. I have never had any problems until now with the previous thread, so I am baffled.

Just a thought but could it have been the physical spool and the thread catching on that little notch? Glad to hear it is working now

When you thread your machine, make sure the pressure foot is UP. When then foot is down, the tension discs are closed. The thread lies along side them instead of inside them. Old saying I have heard since a child and recently heard in a class: " If the Problem on the top, the issue is on the bottom. If the problem is on the bottom, the issue is on the top."

That's good to hear, glad it is resolved. Another idea to keep in the back of my mind, change both top and bottom thread

Make sure your bobbin is in correctly. This happened to me very recently. I scratched my head over what the problem could be, did all that should be done to check tension problems, etc. Then I took the bobbin out and replaced it with a new one - that I put in the right way. Problem solved.

I bought a Bernina Activa 230. I used it for several years and started to have tension problems. I took it in to my dealer and they couldn't find anything wrong with it. She told me not to use Coats and Clark's anymore and use Sulky or some more expensive brand. That did the trick. Maybe try a more expensive thread.