At a recent quilt class, it happened to my SIL, and she was told the bobbin wasn't wound tight enough. She discarded the thread on the bobbin, and pressed a finger between the thread and the machine between the spool holder and bobbin winder. It put just enough pressure on the thread that it wound tighter. No more problem!

Pull your bobbin thread up to the top and take a couple stitches in place and then start out. If you don't pull that bobbin thread to the top to start it will wad up underneath.

If this is happening at the beginning of your stitching, you need to hold the threads, bobbin and upper thread until your stitching begins. If it is happening in the middle your bobbin and upper thread are not set equally. In addition, if you are using different weight thread bobbin or upper thread, it will cause a bird's nest.

Good Luck!

All of the above plus thread with the foot up and then be sure to put it down before you start.

I agree if tension is too loose on top you will have nesting underneath.. clean rethread change needle and adjust tension. Or do one thing at a time then try resewing on something eles to see what the problem is so you know in the future. I would try the tension first though, and if it is that I would still clean, and change needles.... hopes this helps

I recently had this same problem when quilting my DGD's quilt. I finally discovered what the problem was. I was using prewound bobbins, and perodically the thread on the bobbin was crossed and became too tight, that caused the tension on the bobbin thread to pull and the top thread to birds nest. Sometimes the thread would pull loose, and sew on ok. I finally figured it out after the bobbin thread broke a few times and I actually saw the crossed thread on it.
Anyone else have problems with prewound bobbins?

I found that I got nests when I didn't hold the top thread tight while threading the top.

Mine was nesting, and it turned out to be the bobbin tension, which I had never changed. So one day I sat down, and did all of the above, and finally the last thing I did was change the tension on the bobbin. It works fine now. So it's probably something simple. Hang in there!

I find if you hang on to the threads when you start to sew it doesn't do that.

Neat way to troubleshoot!!