Normally, if you are getting thread nests on the bottom, there is no tension at all on the top thread. I agree with the advice to make an appointment at your dealer or with the repair man. Have them show you how to thread the machine, or have them watch while you do it. One of the key threading rules is when to have the presser foot up and when to have it down. With my machine, it MUST be UP for the first part of the threading process, then down for the last few steps. This properly seats the thread in the tension disks.