This may also apply to other Brother machines that have auto knot tying. I've had some trouble with the knot tying in the past; sometimes it works and sometimes it doesn't. Well, I had a lot of knot tying to do (I'm working on a quilt for my DGS with a long boat that has lots of oars which I am sewing on using a satin stitch). I discovered that you need to leave the upper thread totally unimpeded, that is, not under the presser foot. If you do that, it makes the knot perfectly; if you have the thread under the presser foot, it seems like it has a problem pulling it down to the bottom and making the knot. I believe you can avoid birdnesting if you bring up the bobbin thread, and also leave that unimpeded. My habit is to take both threads and pull them behind and under the presser foot, and that is exactly the opposite of what you need to do. Birdnesting wasn't a problem for me here because I'm just working on the quilt top, and not yet on the quilt sandwich.