This is a top tension problem. Many people think when the birdsnest is on the bottom that Itis the bobbin thread. May I explain why it is the top tension. When the threaded needle goes through your material it needs to connect with bobbin thread to make a stitch. If top tension is loose it will leave extra thread on the bottom of your material. the top tension cannot hold the thread correctly. To help yourself understand this on a sample stitch with two distinct different colors of thread in the bobbin and top. Observe your stitches. If the thread from the top shows too much on the bottom the top tension is too loose. If the tread color from bobbin shows on the top then the top tension is too tight. Most all machines recommend adjusting the top tension only. It gets a bigger problem if you try adjusting the bobbin tension. I am a retired shop owner and have had much experience with this concern. Hope this will help you and others in your upcoming projects and great stitching