It could happen that you have been using your dependable Project Runway sewing machine without incident for months, then suddenly on your present project the thread keeps breaking or knotting.
Before you call the manufacturer for a maintenance check, you may want to check a couple of things first which may solve the problem.
Do verify if the thread you are using is of good quality.
Bargain threads, if you look closely, may have excess lint or loose fibers which can cause of thread breakage or knotting.
Use only quality threads to prevent knotting and breaking; also the lint can cause issues with your sewing machine later on.
Next, recheck how you threaded your Project Runway sewing machine. It would be best if you un-thread and re-thread it making sure to keep the presser foot up.
Upper threading mistakes commonly cause thread knotting at the bottom.
Another factor one has to check is the bobbin and if it is threaded properly. Also the bobbin must be in the case in the right direction.