With the Pfaffs there is a half-way down position for the presser foot. As you lower the presser foot lever, apply a slight pressure towards the back of the machine and you should feel it slip into position about half way down. This engages the tension disks so proper tension is applied to your thread as it goes through the machine. Hope this helps your problem. In my experience, no amount of changing needles or thread will help unless the tension disks are engaged.