Everyone has given good advice. This is what I have learned over the years. If it is nesting on the bottom it is usually your top thread. Mine will jump out of the take up lever sometimes. Another thing is always thread the machine with your foot up. That way the tension disks are opened and the thread will be in the tension disk correctly. If you thread with the foot down the disks are closed and the thread won't lay right. I had been sewing for 35 years before I learned that. It took buying a new machine, taking my machine classes, and that was one of the things they taught.