You may want to check and see if you have a bent needle. Sometimes that will happen the first clump and then hit the wrong place and so it will continue to happen. If that and the above methods don't work, you may need to have the machine serviced. Good Luck.

i have both a Topaz 20 and a Singer heavy duty and it happens on both. It usually is the thread coming out of the uptake lever on the front. Sometimes it doesn't come out but wraps around another time and becomes tighter.

I have a Futura which has a plastic bobbin. When the needle goes down in the wrong place. ( whether I am pulling on the sewing or the machine is racing faster than my hoop.) It nicks the edge of the bobbin . Then when the thread is pulled around the bobbin the thread catches on the nick and pulls it back. Then the next stitch catches both threads and so on till you have what is called a "bird's nest." the problem always gets worse unless you can sand the rough spot off or you get a new bobbin. Oh what I would give for a complete metal bobbin. Hope this helps.

This recently happened to me as well and the thread had caught on the top of the machine. I thought I was going along great until the dreaded "thumping" noise.

One thing I always do when having a problem is retread my machine , and also make sure bobbin is inserted correctly

Mine does this when my bobbin gets low but also when I get too many fuzz bunnies in the bobbin area.

I have never used them but I've seen them advertised - the Bobbin Genie. Maybe someone else on the board can tell you whether they would work.

I have found that pulling up my bobbin thread at the beginning of the stitching and hold the bobbing thread and upper thread together keeps that from happening most of the time. I used to think it was an "old wives tale". But, I started doing it and it works for me.

Remember when there is a problem with the underside of your stitches it is usually the upper thread and not the bobbin PaperPrincess is most likely right in what she said. Check all threading and make sure nothing has changed.

When this happened on my Brother machine, it was a threading problem on the top.
This just recently happened with my Elna and it was my bobbin case... which was a surprise to me. I ended up taking it all apart and found that the bobbin case had turned just slightly so it wasn't seating properly. I don't know how it happened, or why, but I got it seated correctly and it's been working fine since.